tools/thermal: tmon: support cross-compiling

We might want to prepare CFLAGS outside of this Makefile, so don't
overwrite its initial value.

Then, support $(CROSS_COMPILE), so we can use a cross-compile toolchain.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

tools/thermal/tmon/Makefile

@@ -2,8 +2,8 @@ VERSION = 1.0
 
 BINDIR=usr/bin
 WARNFLAGS=-Wall -Wshadow -W -Wformat -Wimplicit-function-declaration -Wimplicit-int
-CFLAGS= -O1 ${WARNFLAGS} -fstack-protector
-CC=gcc
+CFLAGS+= -O1 ${WARNFLAGS} -fstack-protector
+CC=$(CROSS_COMPILE)gcc
 
 CFLAGS+=-D VERSION=\"$(VERSION)\"
 LDFLAGS+=