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.

boot.c

@@ -22,7 +22,7 @@ extern __xdata struct machine_runtime machine_detected;
 extern __xdata uint8_t sfr_data[4];
 extern volatile __xdata uint32_t ticks;
 extern __xdata uint8_t tx_seq;
-extern __code uint8_t * __code hex;
+extern __code const uint8_t * __code const hex;