autofs: add patch to force result of procfs check

autofs: add patch to force result of procfs check

The autofs configure script verifies on the build system if /proc is
provided by a Linux system. This might fail if /proc is not mounted,
but does ont mean that procfs will not be available in the target.

Therefore, this commit adds a patch improving the AF_LINUX_PROCFS
autoconf macro to allow passing an autoconf cache variable. autofs.mk
then uses that to force the fact that /proc will be available on the
target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>