We've started pushing init.insmod.*.cfg into the kernel prebuilts, and
installing it to /vendor_dlkm/etc, but it may not be available on all
prebuilt variants (e.g., kernel 5.10).
Here, we:
1. conditionally install our common init.*cfg to /vendor_dlkm (only if
$(TARGET_KERNEL_DIR) doesn't have a copy)
2. point init at /vendor_dlkm/etc
When we've cycled completely off of kernel 5.10, we can drop #1.
Bug: 369686096
Test: build, boot
Flag: EXEMPT refactor
Change-Id: Iff56b163d332d6226c05ff56f7838b3ad9806674
e87bccccca