malta: restore "be" subtarget from being source-only

The malta "be" and "le" subtargets share the arch with many devices of
ar71xx and ramips.  They can be handy for testing, debugging, trying
both snapshot and release builds.

This commit makes the "be" subtarget part of the ci and release builds

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>