ast/BBHandler/build: Add support for removing tasks (deltask)

Back in the depths of time we did support task removal. In the pre
AST days it was nearly impossible to continue supporting it, it wasn't
used so it was dropped. With the modern codebase we can easily now support
deltask and it would be very useful within the metadata since it can
massively simplify dependency trees.

As an example, a core-image-sato had 47703 inter task dependencies before
this patch and a patch to native.bbclass, afterwards with the noexec tasks
deleted, we had 29883. Such a significant simplification is worthwhile
and justifies adding a deltask operation to the system.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
build/ast: Create strong task add/del API in bb.build

Currently its near impossible to control task addition/deletion from
metadata context. This adds stong add/deltask API to bb.build
which is traditionally where it resided. The rather broken
remove_tasks function was removed, it didn't appear to do anything
useful or have any users.

This allows us to clean up hacks currently in use in metadata and use
standard API for it instead.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
fetch2/git: Improve handling of unresolved names verses branches

Currently the fetcher doesn't distinguish between names that the fetcher
needs to resolve verses branch names that the user specified.

This meant that if you specify a tag and a branch, the fetcher broke. This
separates the two so that the branch name is preserved and can be used in
appropriate places.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bb.fetch2.git: reuse basecmd attribute

The basecmd is initialized in urldata_init; there's no need redoing that
work.

Signed-off-by: Olof Johansson <olof.johansson@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>