kinetis_ke: Switch to standard build variables

kinetis_ke: Switch to standard build variables

Instead of using ARM_ prefixed variables and an "arm" target,
use CROSS_COMPILE, AS, OBJCOPY. This requires to switch from ?= to =
to avoid the host assembler getting invoked.

This allows to handle kinetis_ke subdirectory like fm4 and xmc1xxx.

Change-Id: I7ea0bf119f6c4716f4d6002794004730af49eef4
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3505
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>