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.h

@@ -19,7 +19,7 @@ void flash_read_security(void);
 void flash_sector_erase(void);
 void flash_read_bulk(__xdata uint8_t *dst);
 void flash_write_bytes(__xdata uint8_t *ptr);
-__code char* get_flash_size_str(void);
+__code const char* get_flash_size_str(void);
 
 #endif