lib/Makefile.inc: fix hex-to-bin conversion of SPD files

This fixes the hex-to-bin conversion command, used to generated binary
SPD files from hexdumps.

An issue that only appeared on one of my systems, where conversion of
'01 02 03' to binary resulted in \x01\x32\x03 instead of \x01\x02\x03:

for c in 01 02 03; do printf $(printf '\%o' 0x$c); done | xxd -g 1
00000000: 01 32 03                                         .2.

The reason for this was that the syntax in lib/Makefile.inc is wrong,
because the backslash must be escaped due to chaining two printf
commands.

Change-Id: I36b0efac81977e95d3cc4f189c3ae418379fe315
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45207
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/lib/Makefile.inc b/src/lib/Makefile.inc
index 4ce133a..e23b9de 100644
--- a/src/lib/Makefile.inc
+++ b/src/lib/Makefile.inc
@@ -362,7 +362,7 @@
 $(LIB_SPD_BIN): $(LIB_SPD_DEPS)
 	for f in $(LIB_SPD_DEPS); \
 	 do for c in $$(cat $$f | grep --binary-files=text -v ^#); \
-	  do printf $$(printf '\%o' 0x$$c); \
+	  do printf $$(printf '\\%o' 0x$$c); \
 	 done; \
 	done > $@