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.
Merged-In: I929288b218761a4637a2a2ef0679d3ff85c70731
Change-Id: I929288b218761a4637a2a2ef0679d3ff85c70731
(cherry picked from commit 9e95beab3154bd62e0a4bc48853c0198c59800f3)
diff --git a/core/Makefile b/core/Makefile
index 9aae8ca..6138497 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1870,7 +1870,7 @@
 endif
 endif
 
-INTERNAL_AVB_MAKE_VBMETA_IMAGE_ARGS += --padding_size 4096
+BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS += --padding_size 4096
 
 # Add kernel cmdline descriptor for kernel to mount system.img as root with
 # dm-verity. This works when system.img is either chained or not-chained: