Fix unnecessarily creating a temporary directory

mk_build_dir was executed twice in ramdisk-image-create.

Change-Id: I93cf83775fb5e7ba3e6b84643f723568a2420305
Correct misuse of return in ramdisk-image-create

Replaced return with exit.

Change-Id: Iee26177cdb58af67b6919da12456debfe5940105
Merge "Fix unnecessarily creating a temporary directory"
Merge "Correct misuse of return in ramdisk-image-create"