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.

httpd/httpd.c

@@ -22,9 +22,9 @@
 extern volatile __xdata uint8_t sfr_data[4];
 extern volatile __xdata uint32_t ticks;
 extern __xdata uint8_t cmd_capture;	/* owned by rtlplayground.c, see write_char_no_syslog() */
-extern __code uint8_t * __code hex;
-extern __code struct f_data f_data[];
-extern __code char * __code mime_strings[];
+extern __code const uint8_t * __code const hex;
+extern __code const struct f_data f_data[];
+extern __code const char * __code const mime_strings[];
 extern __xdata struct flash_region_t flash_region;
 extern __xdata uint32_t flash_size;
 
@@ -127,7 +127,7 @@ uint8_t find_entry(__xdata uint8_t *e)
 }
 
 
-bool is_word(__xdata uint8_t *xdata_str_p, __code uint8_t * __xdata code_str_p)
+bool is_word(__xdata uint8_t *xdata_str_p, __code const uint8_t * __xdata code_str_p)
 {
 	uint8_t u, c;
 
@@ -149,7 +149,7 @@ bool is_word(__xdata uint8_t *xdata_str_p, __code uint8_t * __xdata code_str_p)
 
 
 /* name must be lower-case, starting with the '\n' of the previous line's end */
-__xdata uint8_t *header_value(__xdata uint8_t *p, __code uint8_t *name)
+__xdata uint8_t *header_value(__xdata uint8_t *p, __code const uint8_t *name)
 {
 	uint8_t u, c;
 

httpd/page_impl.c

@@ -30,7 +30,7 @@ extern __xdata uint16_t slen;
 extern __xdata uint16_t management_vlan;
 extern __xdata uint16_t cont_len;
 extern __xdata uint32_t cont_addr;
-extern __code uint8_t * __code hex;
+extern __code const uint8_t * __code const hex;
 extern __xdata uip_ipaddr_t uip_hostaddr, uip_draddr, uip_netmask;
 
 extern __xdata uint8_t sfr_data[4];
@@ -47,8 +47,8 @@ extern __xdata char sfp_module_model[2][17];
 extern __xdata char sfp_module_serial[2][17];
 extern __xdata uint8_t sfp_options[2];
 
-__code uint8_t * __code HTTP_RESPONCE_JSON = "HTTP/1.1 200 OK\r\nConnection: close\r\nContent-Type: application/json\r\n\r\n";
-__code uint8_t * __code HTTP_RESPONCE_TXT = "HTTP/1.1 200 OK\r\nConnection: close\r\nContent-Type: text/plain\r\n\r\n";
+__code const uint8_t * __code const HTTP_RESPONCE_JSON = "HTTP/1.1 200 OK\r\nConnection: close\r\nContent-Type: application/json\r\n\r\n";
+__code const uint8_t * __code const HTTP_RESPONCE_TXT = "HTTP/1.1 200 OK\r\nConnection: close\r\nContent-Type: text/plain\r\n\r\n";
 
 // Convert uint8_t to ascii HEX char push on html-buffer.
 void charhex_to_html(char c)
@@ -127,7 +127,7 @@ void itoa16_html(uint16_t v) /* sufficient for VLAN IDs (max 4094) */
 	char_to_html('0' + (v % 10));
 }
 
-void string_to_html(__code char *s)
+void string_to_html(__code const char *s)
 {
 	while (*s) char_to_html(*s++);
 }

httpd/page_impl.h

@@ -3,7 +3,7 @@
 
 #include <stdbool.h>
 
-extern __code uint8_t * __code HTTP_RESPONCE_TXT;
+extern __code const uint8_t * __code const HTTP_RESPONCE_TXT;
 
 bool send_counters(uint8_t phys_port);
 void send_status(void);