it is no longer hardcoded just to be handled for uefiextract.

it is now defined as cmakedir in target.cfg, for a single or
multi tree project. if multi tree, it is applied to the specific
tree, and has to be defined per tree

the way it works is: as per cmakelist, a project will define
which directory is to be built, and it will then generate
a makefile in the main source tree (the build tree in cmake
language, where the main CMakeLists.txt file exists)

when the makefile has been generated, the project is then treated
like any other project. the way cmake works, if a makefile has
already been generated by it, in a given directory, running it
again will fail and not affect anything; if it fails but the
makefile doesn't exist, then something is wrong, but if the
makefile does exist, then it's all fine and nothing happens

at present, this is only used for uefiextract, which is part
of src/uefitool

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