I'd better set CROSS_COMPILE in the environment than change my path,
so CROSS_COMPILE is conditionally assigned.

The revision is better "git describe --dirty" than "git rev-parse". The output
is a short beautiful string like "wrpc-v2.0-13-g09a736d5"
with an optional "-dirty" trailer, to show we compiled a non-committed
change.

Finally, to execute commands we dont $(shell) at toplevel, but have
them in the build rule.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>