Makefile: Prettify logging

Replace
	GEN  config-all-devices.mak
by
	GEN   config-all-devices.mak

Like this, the logging output is column aligned.
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>