classes: Print bitbake branch and revision

There is no bitbake revision when it is in a separate repo, then we have no way
to know which version of bitbake is used from the log, this patch always prints
it.

* console-latest.log
bitbake              = "master:165368bad152a14b32b5216b938aa4b915d72a70"
meta
meta-poky
meta-yocto-bsp       = "master:165368bad152a14b32b5216b938aa4b915d72a70"

* buildhistory/metadata-revs
bitbake           = master:165368bad152a14b32b5216b938aa4b915d72a70 -- modified
[snip]

* buildhistory/metadata-revs
-----------------------
Build Configuration:  |
-----------------------
DISTRO = poky
DISTRO_VERSION = 5.0+snapshot-165368bad152a14b32b5216b938aa4b915d72a70
-----------------------
Bitbake Revision:      |
-----------------------
bitbake           = master:165368bad152a14b32b5216b938aa4b915d72a70 -- modified
-----------------------
Layer Revisions:      |
-----------------------
[snip]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>