kbuild: fix infinite make recursion
kbuild: fix infinite make recursion

Jan Engelhardt <f383ed465f890d2e8ea864dd0c28cf8ec50a8942@computergmbh.de> reported:
You can cause a recursion in kbuild/make with the following:

make O=$PWD kernel/time.o
make mrproper

Of course no one would use O=$PWD (that's just the testcase),
but this happened too often:

/ws/linux/linux-2.6.23$ make O=/ws/linux/linux-2.6.23 kernel/time.o
(Oops - should have been O=/ws/linux/obj-2.6.23!)

Fixed by an explicit test for this case - we error
out if output directory and source directory are the same.

Tested-by: Jan Engelhardt <f383ed465f890d2e8ea864dd0c28cf8ec50a8942@computergmbh.de>
Signed-off-by: Sam Ravnborg <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@ravnborg.org>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs