Support non-NXP setups

[landswellsong]

1. NXP-related bbappends moved to dynamic layers so they aren't picked up if the corresponding layers aren't active (allows seamless builds on other platforms).
2. Option to disable building webui with a PACKAGECONFIG option. It's on by default so the existing builds shouldn't change.
3. poco branches corrected for both scarthgap and kirkstone, but the fixes are already upstreamed in meta-openembedded so you might want to update that and then drop this commit/remove the bbappend.
4. The kernel recipes auto apply *.cfg fragments mentioned in SRC_URI so I think there's no need to use DELTA_KERNEL_DEFCONFIG and it is much cleaner.