fs_config: error on multiple specifications of android_filesystem_config.h

Multiple files added to TARGET_ANDROID_FILESYSTEM_CONFIG_H will build,
however the tooling will only respect the first header found. The
sanity check in the makefile will only check the first path found for
proper naming convention. One can end up with a successful build that
does not respect all files in the TARGET_ANDROID_FILESYSTEM_CONFIG_H.

Check that one and only one item is set to
TARGET_ANDROID_FILESYSTEM_CONFIG_H variable.

Output:
*** Multiple fs_config files specified, see "device/lge/bullhead/two/android_filesystem_config.h device/lge/bullhead/one/android_filesystem_config.h"..

Change-Id: I70a43f44713b273cb9a78f54ff16854b9e11af4a
Signed-off-by: William Roberts <william.c.roberts@intel.com>