lbmk: use 2-level directory structure in script/

as opposed to the current 3-level structure.

recent build system simplifications have enabled
this change, thus:

./build fw coreboot -> ./build roms
./build fw grub -> ./build grub
./build fw serprog -> ./build serprog
./update project release -> ./update release
./update project trees -> ./update trees
./update vendor download -> ./vendor download
./update vendor inject -> ./vendor inject

alper criticised that the commands were too long,
so i made them shorter!

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