greybus: build: android: fix strip module build step

Currently, the greybus module .ko files are quite large and the
following error was observed during Android build for each greybus module:
strip: Unable to recognise the format of the input file `<module>`

Fix the strip build step by replacing the undefined KERNEL_TOOLCHAIN_PATH
variable with the GREYBUS_CC_PREFIX variable.  Also used as the
CROSS_COMPILER value for the module make.

Signed-off-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>