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_init.c

@@ -9,7 +9,7 @@
 #include "boot.h"
 
 extern __xdata uint8_t sfr_data[4];
-extern __code struct machine machine;
+extern __code const struct machine machine;
 extern __xdata struct machine_runtime machine_detected;
 
 #pragma codeseg BANK2