It appears that Espressif patched the open source xtensa GDB port in
order to build their old GDB executable and their old gdbstub (basically
removing any register in a generic xtensa and only leaving those
present in the chip they synthesized).  Their GDBStub also assumed this
behavior.

Unpatched upstream GNU GDB now expects all the registers in
xtensa-config.c to be sent/read on a 'g' command.  Change the GDB stub
to send "xxxxxxxx"s (legal per the spec) for unimplemented registers.
This makes the 'g' response much longer, but it's results are cached
and in an interactive debugger it isn't noticeable.

All functions which are not interrupt or exception called are now in
flash. A small IRAM wrapper enables flash when processing main GDB ops
by calling Cache_Read_Enable_New() and then jumping to the main flash
code.  This seems to work for catching exceptions, data and code breaks,
and Ctrl-C.

The UART ISR handler and exception handler register-saving bits of
code in ASM are still in IRAM.

GDB IRAM usage is now about 670 bytes.

Add some simple GDB documentation to the main tree showing a worked
example.

Adds the definition of `void gdbstub_init()` to <GDBStub.h>

Replace GDBstub.h with the version in the internal/ directory, and
adjust stub code accordingly.  This way, only one copy of a file called
"GDBstub.h" will exist.

Update the gdbcommands and replace the obsolete ESPRESSIF readme with
@kylefleming's version since we're mainly doing serial, not TCP,
connected debugging.

Bump the library rev. number since this is a pretty big functionality
change.

Minor documentation tweak.

Remove the refactoring of pin control and other little things not directly
related to GDB processing.  Should greatly reduce the diff size in uart.c.
Should also remove any register value changes (intended or otherwise)
introduced in the original PR from @kylefleming.

Set the FIFO interrupt to 16 chars when in GDB mode, matching the latest
UART configuration for highest speed.

Comments added to UART.c trying to explain (as best as I understand it)
the changes done to support GDB and how they interact with standard
operation.

Fix the uart_uninit to stop the ISR and then free appropriately.

Fix uart_isr_handle_data (GDB's shim for sending chars to the 8266 app)
to do the exact same thing as the standard UART handler including set
the overflow properly and either discard or overwrite in that case.

Fix serial reception when GDB enabled by enabling the user recv ISR.

Remove commented attributes from gdbstub, leftover from the move to
flash.

General logic cleanup per comments in the PR.

Ensure we also check the UART flags and set the uart status
appropriately when in GDB mode.