Makefile: Add a strip_quotes macro

Change-Id: I465416dd8e4f92f9700253fc031a3933bba93f38
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/16962
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/Makefile b/Makefile
index 2509b58..aa0d3ab 100644
--- a/Makefile
+++ b/Makefile
@@ -16,6 +16,8 @@
 space +=
 comma := ,
 
+strip_quotes = $(strip $(subst ",,$(1))) #"
+
 $(foreach dep,$($(name)-deplibs), \
 	$(eval -include $($(dep)-dir)/config))
 -include $(cnf)