16k: Add 16K variant kernel/ramdisk build targets

This adds two new build variables, BOARD_KERNEL_PATH_16K and
BOARD_KERNEL_MODULES_16K . BOARD_KERNEL_PATH_16K will be copied
to output directory as is.

BOARD_KERNEL_MODULES_16K is a list of modules which will be packed into a
ramdisk and added to output dir. This allows cuttlefish to switch to 16K
kernel at runtime without adding too much overhead, as ramdisk building
takes very little time.

Bug: 253827893
Test: m kernel_16k ramdisk_16k

Change-Id: Ibb36ba4c3399b3f193795aea6597a4aa535f1bec