tools/thermal/tmon: Add cross compiling support

Default to prefixed pkg-config when crosscompiling, this matches what
other parts of the tools/ directory already do.

[dlezcano] : Reworked description
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/31302992.qZodDJZGDc@devpool47

tools/thermal/tmon/Makefile

@@ -10,7 +10,7 @@ override CFLAGS+= $(call cc-option,-O3,-O1) ${WARNFLAGS}
 # Add "-fstack-protector" only if toolchain supports it.
 override CFLAGS+= $(call cc-option,-fstack-protector-strong)
 CC?= $(CROSS_COMPILE)gcc
-PKG_CONFIG?= pkg-config
+PKG_CONFIG?= $(CROSS_COMPILE)pkg-config
 
 override CFLAGS+=-D VERSION=\"$(VERSION)\"
 TARGET=tmon