kernel: fix broken initramfs images for Separate cpio

kernel: fix broken initramfs images for Separate cpio

Commit 0137fbd74bff ("kernel: skip rebuilding kernel with
ROOTFS_INITRAMFS_SEPARATE") had a logic error error and didn't account
that the generic initramfs is still needed to be built to enable support
for the cpio compression and other config specific to initramfs. With
that commit we completely skip rebuilding the kernel with those new
options.

To better handle this, skip kernel build ONLY when we are handling Per
Device Rootfs, permitting the first generic build to be actually called
with the new options.

Fixes: 0137fbd74bff ("kernel: skip rebuilding kernel with ROOTFS_INITRAMFS_SEPARATE")
Link: https://github.com/openwrt/openwrt/pull/15912
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>