Align size of vbmeta image to 4k boundary

Adding option "--padding_size 4096" in BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS
instead of INTERNAL_AVB_MAKE_VBMETA_IMAGE_ARGS. The former setting will
be propogated to `make dist` while the latter is only used for `make`.

Bug: 38454093
Test: `make dist` and check vbmeta.img is being padded.
Change-Id: I929288b218761a4637a2a2ef0679d3ff85c70731