lib.sh: new function mk() to handle trees in bulk

single-tree projects cannot be handled in bulk, e.g.
./mk -f project1 project2 project3

that is still the case, from the shell, but internally
it is now possible:
mk -f project1 project2 project3

mk() is a function that simply handles the given flag,
and all projects specified.

it does not handle cases without argument, for example
you cannot do:
mk -f

arguments must be provided. it can be used internally,
to simplify cases where multiple single-tree projects
must be handled, but *also* allows multi-tree projects
to be specified, without being able to actually handle
trees within that multi-tree project; so for example,
you can only specify coreboot, and then it would run
on every coreboot tree.

Signed-off-by: Leah Rowe <leah@libreboot.org>