bitbake: gitannex.py: Add Git Annex support

This add a Git Annex backend which reuses the Git fetcher code; it
allows managing files with git, without checking the file contents
into git, being useful when dealing with files larger than git can
currently easily handle, whether due to limitations in memory, time,
or disk space.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake: fetch2/git: add description for nobranch

Signed-off-by: Zhenhua Luo <zhenhua.luo@freescale.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>