Exclude board cmdline/dtb/pagesize/base in generic boot image

When building with BOARD_USES_GENERIC_KERNEL_IMAGE, even if BUILDING_VENDOR_BOOT_IMAGE
is not set, do not include board-specific cmdline, dtb, page size, and base in the
generic boot image.

This change drops buildvariant=* in the cmdline of the generic boot
image

Bug: 156098440
Test: manual. Deliberately set BOARD_KERNEL_CMDLINE for aosp_arm64 and
      ensure it doesn't go into the boot image.
Change-Id: I846f600058a4a9b349d55c9773d6dd81bbe49312