Makefile: fix readelf usage

Some OS (like OS X) do not provide a generic readelf. We should enforce to use
the toochain provided readelf instead, to do so use $(CROSS_COMPILE)readelf.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Tested-by: Lubomir Popov <lpopov@mm-sol.com>