test/stp: build the simulator against the firmware headers

test/stp/shim held hand-written copies of the headers the STP code
includes, and they had drifted: the uIP buffer was 600 bytes instead of
1556, struct machine had three fields, port_ingress_filter_get()
returned a plain uint8_t. The simulator now compiles rtl837x_stp.c and
rtl837x_stp_cli.c the way test/ compiles firmware sources, with the real
headers and sdcc_shim.h forced in, and with the same warnings, including
-Wno-pointer-sign for the string literals passed to cmd_compare(). It
stays on -std=c11: in gnu11 mode glibc defines LITTLE_ENDIAN, which
uip-conf.h names as the byte order, and HTONS() stops swapping. Its own
print_string() and cmd_compare() take the firmware's argument types.

test/stp/stp_sim.c

@@ -29,7 +29,7 @@ uint8_t err_status;
 uint8_t atoi_results_u8;
 
 static int verbose;
-void print_string(char *s) { if (verbose) fputs(s, stdout); }
+void print_string(const char *s) { if (verbose) fputs(s, stdout); }
 void print_byte(uint8_t b) { if (verbose) printf("%02x", b); }
 void print_short(uint16_t v) { if (verbose) printf("%04x", v); }
 void print_long(uint32_t v) { if (verbose) printf("%08x", v); }
@@ -37,7 +37,7 @@ void write_char(char c) { if (verbose) putchar(c); }
 void itoa(uint8_t v) { if (verbose) printf("%u", v); }
 void print_reg(uint16_t reg) { (void)reg; }
 
-uint8_t cmd_compare(uint8_t start, uint8_t *cmd)
+uint8_t cmd_compare(uint8_t start, const uint8_t *cmd)
 {
 	if (start >= cmd_words_len)
 		return 0;