greybus: build: android: replace Linaro build specific logic with AOSP equivalents

When using greybus build in a different Android setup, it was noted that
several portions of this makefile rely on Linaro specific build items.
Replace these with more generic build steps.

- ANDROID_64 is only defined by Linaro build tasks, use TARGET_ARCH to
  establish ARCH= parameter for greybus build
- KERNEL_TOOLS_PREFIX is only defined by Linaro build tasks.  AOSP has
  a near equivalent variable: TARGET_TOOLS_PREFIX
- build-greybus was dependant on subtask: android_kernel a task defined
  only by Linaro build tasks.  Replace with a generic dependancy to
  the kernel binary located in $OUT (INSTALLED_KERNEL_TARGET).
  End result is the same: kernel must be built before greybus modules

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