apply-patches.sh: detect missing patches

The "patch" command returns an error code only if patches fail
to apply. Therefore the pipleline "cat <patchfile> | patch ..."
does not fail, even if <patchfile> is missing. Fix this by
adding an explicit check for patch file existence.

Based on feedback from buildroot mailing list, also change the
existing check for unsupported patch format into a fatal error.

Signed-off-by: Ralph Siemsen <ralphs@netwinder.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>