hexstrtobin: Add a library function to decode ASCII hex into binary

This function will turn a string of ASCII hex characters into an array
of bytes.  It will ignore any non-ASCII-hex characters in the input
string and decode up to len bytes of data from it.

This can be used for turning MAC addresses or UUID strings into binary
for storage or further processing.

Sample usage:
  uint8_t buf[6];
  hexstrtobin("00:0e:c6:81:72:01", buf, sizeof(buf));
  acpigen_emit_stream(buf, sizeof(buf));

Change-Id: I2de9bd28ae8c42cdca09eec11a3bba497a52988c
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/14837
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/lib/Makefile.inc

@@ -114,6 +114,7 @@ ramstage-y += cbfs.c
 ramstage-$(CONFIG_COMMON_CBFS_SPI_WRAPPER) += cbfs_spi.c
 ramstage-y += lzma.c lzmadecode.c
 ramstage-y += stack.c
+ramstage-y += hexstrtobin.c
 ramstage-y += wrdd.c
 ramstage-$(CONFIG_CONSOLE_CBMEM) += cbmem_console.c
 ramstage-$(CONFIG_BOOTSPLASH) += jpeg.c