Replace overrides implemented to install additional grub modules by simply appending to the GURB_BUILDIN variable.
measured-image-bootimg: Rename directory holding bootloader configs.

This allows the debug version of the image to find these files.
grub.cfg: Add new menuentry to boot w/o tboot.
init-boot-tpm.sh: Replace calls to MKDIR with small function.

This gets rid of complaining about directories that already exist etc.