toasterconf: remove master as a branch option from fido release

toasterconf: remove master as a branch option from fido release

Toaster isn't designed to be forward compatible. As such,
a release cannot build releases newer then it.

Particularly, "fido" cannot build "master", so we remove
"master" from the list of supported releases in "fido"

[YOCTO #8154]

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Joshua Lock <joshua.lock@collabora.co.uk>