scripts/kconfig/Makefile: Fix KBUILD_DEFCONFIG check when building with O=

My recent commit d2036f30 ("scripts/kconfig/Makefile: Allow
KBUILD_DEFCONFIG to be a target"), contained a bug in that when it
checks if KBUILD_DEFCONFIG is a file it forgets to prepend $(srctree) to
the path.

This causes the build to fail when building out of tree (with O=), and
when the value of KBUILD_DEFCONFIG is 'defconfig'. In that case we will
fail to find the 'defconfig' file, because we look in the build
directory not $(srctree), and so we will call Make again with
'defconfig' as the target. From there we loop infinitely calling 'make
defconfig' again and again.

The fix is simple, we need to look for the file under $(srctree).

Fixes: d2036f30

 ("scripts/kconfig/Makefile: Allow KBUILD_DEFCONFIG to be a target")
Reported-by: Olof Johansson <olof@lixom.net>
Acked-by: Michal Marek <mmarek@suse.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>