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.

installer/installer.c

@@ -30,7 +30,7 @@
 __xdata uint8_t buffer[0x1000];
 __xdata uint8_t dio_enabled;
 
-__code uint8_t * __code hex = "0123456789abcdef";
+__code const uint8_t * __code const hex = "0123456789abcdef";
 
 void isr_timer0(void) __interrupt(1)
 {
@@ -67,7 +67,7 @@ void write_char(char c)
 }
 
 
-void print_string(__code char *p)
+void print_string(__code const char *p)
 {
 	while (*p)
 		write_char(*p++);