bitbake.conf: sort MIRROR list, add missing SAMBA_MIRROR

As is the pattern with all lists, put the list of MIRRORs in
alphabetical order, and add an entry for SAMBA_MIRROR as it appears in
the samba recipe from the meta-networking layer.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>