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/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)
 {