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.

test/env_tables.c

@@ -90,7 +90,7 @@ struct uip_eth_addr uip_ethaddr = { .addr = { 0x02, 0x11, 0x22, 0x33, 0x44, 0x55
 
 /* ---- leaf calls into subsystems not under test ---- */
 void     flash_read_bulk(uint8_t *dst) { (void)dst; }
-char    *get_flash_size_str(void) { return (char *)"2M"; }
+const char *get_flash_size_str(void) { return "2M"; }
 uint8_t  sfp_read_reg(uint8_t slot, uint8_t reg) { (void)slot; (void)reg; return 0; }
 bool     gpio_pin_test(uint8_t pin) { (void)pin; return false; }
 void     phy_read(uint8_t phy_id, uint8_t dev_id, uint16_t reg) { (void)phy_id; (void)dev_id; (void)reg; }