pm: cpupower: Makefile: Allow overriding cross-compiling env params

Allow overriding the cross-comple env parameters to make it
easier for Yocto users. Then cross-compiler toolchains to build
cpupower with only two steps:
- source (toolchain path)/environment-setup-armv8a-poky-linux
- make

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: MrVan

tools/power/cpupower/Makefile

@@ -86,12 +86,12 @@ INSTALL_SCRIPT = ${INSTALL} -m 644
 # If you are running a cross compiler, you may want to set this
 # to something more interesting, like "arm-linux-".  If you want
 # to compile vs uClibc, that can be done here as well.
-CROSS = #/usr/i386-linux-uclibc/usr/bin/i386-uclibc-
-CC = $(CROSS)gcc
-LD = $(CROSS)gcc
-AR = $(CROSS)ar
-STRIP = $(CROSS)strip
-RANLIB = $(CROSS)ranlib
+CROSS ?= #/usr/i386-linux-uclibc/usr/bin/i386-uclibc-
+CC ?= $(CROSS)gcc
+LD ?= $(CROSS)gcc
+AR ?= $(CROSS)ar
+STRIP ?= $(CROSS)strip
+RANLIB ?= $(CROSS)ranlib
 HOSTCC = gcc
 MKDIR = mkdir