Revert "kbuild: remove libelf checks from top Makefile"

This reverts commit 0d989ac2.

Android kernel isn't ready for this change yet.  Here's why:

There are a couple of different methods for linking the Android
kernel presently, hermetically and not.  When *not* building
hermetically, the kernel is linked against locally installed
libraries (OpenSSL, LibElf, etc), usually installed by the distro.
Since these are usually fully fledged versions, they're usually
sufficient for any possible combination of build options.

However, when building hermetically (the intended default), the kernel
is linked against libraries contained in the `repo` controlled AOSP
repositories.  This is not a problem for the vast majority of the
important builds, but x86 allmodconfig builds require more
functionality than is presently provided.

This revert changes the presence of LibElf to be voluntary again.
Meaning that we do not have to insist on the presence of a locally
installed instance.

Once OpenSSL (currently provided via BoringSSL) is upgraded to a more
featureful offering, we can consider re-inclusion of this patch.

Change-Id: I810139b034e1a6c2db4dc9fa8871279f5edf215d
Signed-off-by: Lee Jones <lee.jones@linaro.org>