[mb2] Replace grep, sed usage with rpmspec

This makes sure all macros are correctly expanded, and
all .spec formats that are supported by RPM can be parsed.

https://wiki.merproject.org/wiki/Packaging_guidelines/ParsingSpecFiles
[mb2] deal with submodule/subtree spec files

we must first cd to the directory in build/install steps before we try do
anything.
[mb2] Modify submodule/subtree spec dealings

mb2 will now parse simple spec %{tags} from the %setup line for
the subtree name. It is still probably a good idea to not use
rpm macros in the spec setup line.

The changedir to the subtree is done in a subshell with the sb2
command.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jollamobile.com>
[mb] deal with subtree spec files

When using tar_git with submodules it is normal to have /%{name} in
%setup line so we need to copy that to the temp.spec as well.

Signed-off-by: Marko Saukko <marko.saukko@jollamobile.com>