Fix init_boot.img build rule with TARGET_NO_KERNEL:=true

If TARGET_NO_KERNEL is true (device has prebuilt boot.img), then the
init_boot.img build arguments wouldn't be propagated correctly.
This is because INTERNAL_INIT_BOOT_IMAGE_ARGS is misplaced within a
`ifneq ($(strip $(TARGET_NO_KERNEL)),true)` conditional block.

Fix this by moving INTERNAL_INIT_BOOT_IMAGE_ARGS initialization into the
`ifeq ($(BUILDING_INIT_BOOT_IMAGE),true)` block, which is closer to its
first use.

Bug: 237630862
Test: m initbootimage
Change-Id: I6c0ee2c4b5d0be77a93311eba714e0ed555ffd25