Version.mk: Build number with local date

It's really annoying building zips with yesterdays date

PS2: Only for HOMEMADE builds
PS3: Fix -u Thats what happens when you only test the first case
PS4: Timezone
PS5: export ROM_BUILDTIME_LOCAL=y to set localtime build

Change-Id: I651d228b14871cc1615c402d0cd93ccce33f93b4