bitbake: unify mirror support and make it independant of the fetcher
This patch serves two purposes. Firstly it unifies the concept of mirrors into
PREMIRRORS and MIRRORS. PREMIRRORS are tried before the SRC_URI defined in the
recipe whereas MIRRORS are tried only if that fails.
The tarball stash was conceptually inline with a PREMIRROR only with special
handling within the wget fetcher and therefore only worked with certain
fetch types.
Secondly the patch removes the need for individual fetch implementations to
worry about mirror handling.
With this patch, the base fetch implementation will first try to use a
PREMIRROR to fetch the desired object, if this fails the native fetch method
for the object will be tried and if this fails will try to fetch a copy from
one of the MIRRORS.
