add BOARD_VENDOR_RAMDISK_KERNEL_MODULES[_LOAD]

To accommodate a future vendor ramdisk, recognize
BOARD_VENDOR_RAMDISK_KERNEL_MODULES and
BOARD_VENDOR_RAMDISK_KERNEL_MODULES_LOAD as a way to specify kernel
modules to be stored on the vendor ramdisk, and the optional override list of
kernel modules to be loaded by first stage init.

For now this will affect the modules loaded on the generic ramdisk,
but will provide a way for vendors to add support now and hopefully not
have to modify it later when the vendor ramdisk is created.

BOARD_RAMDISK_KERNEL_MODULES is renamed to
BOARD_GENERIC_RAMDISK_KERNEL_MODULES, which is not expected to be
modified by vendors.

Bug: 137297791
Change-Id: I05944e004ab5235172da3a15b8d1c6f4c4c021ce