classes/sanity: check for case-sensitive file systems

classes/sanity: check for case-sensitive file systems

Case-insensitive file systems fail during builds in very mysterious ways, such
as mpfr:

  ERROR: patch_do_patch: Not a directory

The problem here being that mpfr has a PATCHES file, so when we try to copy the
patches into ${S}/patches/ it fails.

We can't and won't support case-insensitive file systems so add a sanity check
to abort the build if one is found.

Signed-off-by: Ross Burton <ross.burton@intel.com>