Revert "Revert "kbuild: remove libelf checks from top Makefile""

This reverts commit a2480b4d.

LibELF is now linkable from the Android Kernel build infra thanks to a
recent patch making hermetic the default for all builds.

Bug: 191637574
Fixes: a2480b4d ("Revert "kbuild: remove libelf checks from top Makefile"")
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I6eb8aced5766ea759780d27e82b34256971fdf50