Add support for linking PIC shared object, which only require
linking, using their PLT and GOT lists and don't need any relocation
otherwise.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Add support for relocating local symbols, as specified in the
.rela.dyn section.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Applications can use custom ELF sections for their own purposes, add
a function for finding them.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Export some symbols for loadable modules. Also add an
EXPORT_SYSCALL() helper macro for exporting system calls by their
official names.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Some applications can decide to link their loadable objects for
exactly the same addresses, where they will be loaded. In those cases
local relocations aren't needed any more and can in fact break the
object if applied while the object is in a temporary storage. Add a
parameter to skip such local relocations.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

llext_seek(), llext_read() and llext_peek() are needed outside of the
extension code too, move them to a header.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Add support for running a modular "Hello world" example on Xtensa.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Loadable modules can contain global (not "static") functions, even if
they aren't exported for use by other modules, e.g. when a module is
built from multiple .c files. Such functions are then also included
in link tables and have to be re-linked.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Extend the llext_load() / llext_unload() API to let it be called
repeatedly for the same extension to increment or decrement its
reference counter respectively. We use a mutex to protect the counter
and make both llext_load() and llext_unload() return the use-count to
let the caller identify when the first loading and the last unloading
took place.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

If a function fails it should release all the resources it has
managed to acquire. Fix llext_load() to free memory that it has
allocated in case of an error.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Extensions should be able to selectively export their global symbols.
Add a LL_EXTENSION_SYMBOL() macro for that. Change the present
.sym_tab to be a temporary symbol table of all global symbols in an
extensions, used only during linking for internal purposes. Add a new
.exp_tab symbol table to store symbols, exported by an extension
permanently.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Use an element size explicitly when calculating the array size and
use the calculated size for memset().

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

The llext list should be internal to llext.c, remove its scanning
from shell.c, export a function for that instead.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

llext_list() is an exported function that returns a pointer to the
llext internal extension list. That list should only be accessible
directly inside llext, while holding a lock. Remove the function.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

a new llext object is completely initialised with zeros after
allocation, no need to additionally set members of an embedded into
it array to NULL.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Use an existing mutex to also protect the global llext list.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

When using the LLEXT buffer loader we now avoid copying extensions
from storage to allocated memory by pointing directly into the stored
image. We then also perform linking and relocation in that memory,
which modifies its contents. However, this is impossible if that
storage is read-only. Add a Kconfig flag to distinguish between
writable and read-only storage types. Also use that flag to decide,
whether the extension image in test_llext_simple.c should be defined
as const or not.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>