Fix warnings seen on SDCC 4.6.0 due to missing const qualifiers

Building with SDCC 4.6.0 emits tons of warnings e.g.:

	cmd_parser.c:31: warning 356: object in read-only code space should be const
	cmd_parser.c:36: warning 357: pointer to object in read-only code space should be pointer to const

These of course become fatal when building with --Werror.

Comparing resultant binaries, it seems that a single string is shifted
(deduplication?) but the numbers emitted at the end of the build are
otherwise identical.

rtl837x_flash.c

@@ -171,7 +171,7 @@ void flash_read_uid(void)
 	flash_configure_mmio();
 }
 
-__code char* get_flash_size_str(void)
+__code const char* get_flash_size_str(void)
 {
 	switch (flash_capacity_code) {
 		case 0x12: return "256 KB";