build: show the compiler and the toolchain in the version banner

The boot banner and the version command print the SDCC release and
revision from the compiler's own macros, and the name of the in-tree
toolchain the Makefile built, which is the checksum of sdcc/ and so
changes with every patch in it. A build with CC given on the command
line says "system" instead, since that compiler need not carry the
patches. A plain release and the same release with the patches report
the same revision, so the toolchain line is what tells them apart.

Costs 66 bytes of BANK2.

rtl837x_common.h

@@ -19,6 +19,8 @@
 #define MAC_ADDR_LEN    6
 #define PORT_BIT(p)     ((uint16_t)1 << (p))
 #define NUL			'\0'
+#define STR_(x)		#x
+#define STR(x)		STR_(x)
 
 // Define Port-masks for 9-port devices and 6-port devices
 #define PMASK_9		0x1ff