kernel: add `make kernelxconfig' target

kernelxconfig is identical to kernelconfig except that it launches
xconfig instead of menuconfig. xconfig easier to use if you need to
search the Kconfig tree.

Change-Id: I6d91c7e3efbc9f172b040425f2ec545e3f03b44d
(cherry picked from commit 60ca2b7897b552ce5dee2633fc84906222579763)
1 file changed