Make 'mkbootfs' print a usage message for insufficent args.

Right now, it just prints "no directories to process". There is also no
indication anywhere about the availability of the -f and -d flags.

Also took the opportunity to perform small quality-of-life improvements,
like updating error messages, hiding internal functions.

Test: (cd system/core/mkbootfs && mma) && ./out/host/linux-x86/bin/mkbootfs
Change-Id: Ia81eff0308e6e2686dde979d47c0bf938afbaf82