fetch2: Unify BB_FETCH_PREMIRRORONLY
The fetch2/__init__.py checks whether "BB_FETCH_PREMIRRORONLY" == "1", but
fetch2/git.py and hg.py checks whether it is None, this makes it discontinuous,
and BB_FETCH_PREMIRRORONLY = "0" doens't work as expected in the later case,
so unify it to the previous one. (As BB_NO_NETWORK does).
And also use bb.utils.to_boolean() to make them consistent.
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>