pylibfdt: Allow pkg-config to be supplied in the environment

Some build systems have their own version of the pkg-config tool.
Use a variable for this instead of hard-coding it, to allow for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Mike Frysinger <vapier@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -23,6 +23,7 @@ CFLAGS = -g -Os -fPIC -Werror $(WARNINGS)
 BISON = bison
 LEX = flex
 SWIG = swig
+PKG_CONFIG ?= pkg-config
 
 INSTALL = /usr/bin/install
 DESTDIR =
@@ -119,7 +120,7 @@ SCRIPTS = dtdiff
 # We need both Python and swig to build pylibfdt.
 .PHONY: maybe_pylibfdt
 maybe_pylibfdt: FORCE
-	if pkg-config --cflags python >/dev/null 2>&1; then \
+	if $(PKG_CONFIG) --cflags python >/dev/null 2>&1; then \
 		if which swig >/dev/null 2>&1; then \
 			can_build=yes; \
 		fi; \