software/bios: add target hooks

This allows the target to supply custom target_init() and target_boot()
functions, eg:

    # add custom target specific library to the bios:
    src="libtarget"
    src_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), src))
    builder.add_software_package(src, src_dir)
    builder.add_software_library(src)

litex/soc/software/bios/boot.c

@@ -580,7 +580,7 @@ void netboot(int nb_params, char **params)
 /* Flash Boot                                                            */
 /*-----------------------------------------------------------------------*/
 
-#ifdef FLASH_BOOT_ADDRESS
+#if defined(FLASH_BOOT_ADDRESS) || defined(MAIN_RAM_BASE)
 
 static unsigned int check_image_in_flash(unsigned int base_address)
 {
@@ -603,9 +603,10 @@ static unsigned int check_image_in_flash(unsigned int base_address)
 
 	return length;
 }
+#endif
 
-#if defined(MAIN_RAM_BASE) && defined(FLASH_BOOT_ADDRESS)
-static int copy_image_from_flash_to_ram(unsigned int flash_address, unsigned long ram_address)
+#if defined(MAIN_RAM_BASE)
+int copy_image_from_flash_to_ram(unsigned int flash_address, unsigned long ram_address)
 {
 	uint32_t length;
 	uint32_t offset;
@@ -632,6 +633,7 @@ static int copy_image_from_flash_to_ram(unsigned int flash_address, unsigned lon
 }
 #endif
 
+#if defined(FLASH_BOOT_ADDRESS)
 void flashboot(void)
 {
 	uint32_t length;

litex/soc/software/bios/boot.h

@@ -12,5 +12,8 @@ void flashboot(void);
 void romboot(void);
 void sdcardboot(void);
 void sataboot(void);
+extern void target_init(void) __attribute__((weak));
+extern void target_boot(void) __attribute__((weak));
+extern int copy_image_from_flash_to_ram(unsigned int flash_address, unsigned long ram_address);
 
 #endif /* __BOOT_H */

litex/soc/software/bios/main.c

@@ -57,6 +57,8 @@ static void boot_sequence(void)
 	if (serialboot() == 0)
 		return;
 #endif
+	if (target_boot)
+		target_boot();
 #ifdef FLASH_BOOT_ADDRESS
 	flashboot();
 #endif
@@ -297,6 +299,10 @@ __attribute__((__used__)) int main(int i, char **c)
 	/* Execute  initialization functions */
 	init_dispatcher();
 
+	/* Execute any target specific initialisation (if linked) */
+	if (target_init)
+		target_init();
+
 	/* Execute Boot sequence */
 #ifndef CONFIG_BIOS_NO_BOOT
 	if(sdr_ok) {