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; }

test/stub/html_data.h

@@ -5,7 +5,7 @@
 #define FDATA_DEFS_H
 #include <stdint.h>
 typedef enum mime_type_e { mime_HTML = 0, mime_SVG, mime_ICO, mime_PNG, mime_JS, mime_CSS, mime_TXT } mime_type_t;
-struct f_data { __code char *file; uint32_t start; uint16_t len; mime_type_t mime; uint8_t gzip; };
+struct f_data { __code const char *file; uint32_t start; uint16_t len; mime_type_t mime; uint8_t gzip; };
 #define FDATA_START_login_html		0x00040000UL
 #define FDATA_START_favicon_ico		0x00041000UL
 #endif

test/test_httpd_tx.c

@@ -67,18 +67,18 @@ volatile uint8_t sfr_data[4];
 volatile uint32_t ticks;
 uint8_t cmd_capture;
 uint8_t err_status;
-uint8_t *hex = (uint8_t *)"0123456789abcdef";
+const uint8_t * const hex = (const uint8_t *)"0123456789abcdef";
 uint16_t crc_value;
-uint8_t *HTTP_RESPONCE_TXT = (uint8_t *)"HTTP/1.1 200 OK\r\n\r\n";
+const uint8_t * const HTTP_RESPONCE_TXT = (const uint8_t *)"HTTP/1.1 200 OK\r\n\r\n";
 uint32_t flash_size = 0x80000;
 uint8_t flash_buf[FLASH_BUF_SIZE];
 uint8_t rx_headers[16];
 struct flash_region_t flash_region;
 
-char *mime_strings[] = { "text/html", "image/svg+xml", "image/x-icon",
+const char * const mime_strings[] = { "text/html", "image/svg+xml", "image/x-icon",
 			 "image/png", "text/javascript", "text/css", "text/plain" };
 
-struct f_data f_data[] = {
+const struct f_data f_data[] = {
 	{ FILE_NAME, FILE_START, FILE_LEN, mime_HTML, 0 },
 	{ 0, 0, 0, mime_HTML, 0 },
 };
@@ -92,7 +92,7 @@ void flash_read_bulk(uint8_t *dst)
 void flash_init(uint8_t enable_dio) { (void)enable_dio; }
 void flash_sector_erase(void) { }
 void flash_write_bytes(uint8_t *ptr) { (void)ptr; }
-char *get_flash_size_str(void) { return "512 kB"; }
+const char *get_flash_size_str(void) { return "512 kB"; }
 void crc16_bank1(uint8_t *v) { (void)v; }
 void reset_chip(void) { }
 void delay(uint16_t t) { (void)t; }
@@ -124,7 +124,7 @@ uint16_t strtox(uint8_t *dst, const char *s)
 	return n;
 }
 
-void memcpyc(uint8_t *dst, uint8_t *src, uint16_t len) { memcpy(dst, src, len); }
+void memcpyc(uint8_t *dst, const uint8_t *src, uint16_t len) { memcpy(dst, src, len); }
 
 bool strstart(const uint8_t *a, const uint8_t *b)
 {