kbuild: Remove reference to uninitialised variable

Verbose output variable is unnecessary because the command's echo is
already surpressed. Additionally because the block defines skip-makefile
the variable Q is not defined within the makefile, which can cause
problems if Q is defined in the users environment.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>