kernel: Support pixel style vendor_boot modules

Mini documentation:
List the names of boot modules in BOOT_KERNEL_MODULES to add them
to vendor_boot.

All modules still end up in their primary location (normally
vendor/lib/modules). Boot modules will be kept here as duplicates.

The list of boot modules is then depmod'ed and copied to the
vendor ramdisk.

The file modules.load is created from
BOARD_$(IMAGE)_KERNEL_MODULES_LOAD as the ordered list of modules
to be loaded form that location, either parsed by a modprobe script
or by init itself (only boot modules)
When using vendor_boot modules, vendor/lib/modules/modules.load
should list all modules but boot modules.

Also don't clear the output directory, because one might want to
push a modules.blocklist there.

Change-Id: I89a4aad42a6585a572812f1d7e89f5d78931e1e6