.gitignore: add files produced by b4

b4 utility [1] is introduced by Linux Kernel developers and used to
fetch patches and patch series from lore.kernel.org and is proven
to be useful for U-Boot development. Detailed usage of the tool can be
read under post from the original author [2].

This tool fetches files from the list and populates the source folder
with additional files (*.cover and *.mbx) which are not ignored by git
and shown as newly added files.

Add those file patterns into .gitignore file, so they can be safely
skipped during changes attestation.

Link: [1]: https://pypi.org/project/b4/
Link: [2]: https://people.kernel.org/monsieuricon/introducing-b4-and-patch-attestation


Signed-off-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Tom Rini <trini@konsulko.com>