lib/oe/patch.py: Prefer "git am" over "git apply" when applying git patches

lib/oe/patch.py: Prefer "git am" over "git apply" when applying git patches

It is better to use "git am" when possible to preserve the commit messages and
the mail format in general for patches when those are present. A typical use
case is when developers would like to keep the changes on top of the latest
upstream, and they may occasionally need to rebase. This is not possible with
"git diff" and "diff" generated patches.

Since this is not always the case, the fallback would be the "git apply"
operation which is currently available.

(From OE-Core rev: 3a14b0943731822905e6d45b13d08a6e8237e2fe)

Signed-off-by: Laszlo Papp <lpapp@kde.org>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>