build: tasks: kernel: Rework kernel fragments handling

* Since some time you can specify multiple configs when you prepare .config and they'll get merged to main one
* To use it inline:
TARGET_KERNEL_CONFIG := main_defconfig fragment1.config fragment2.config ...
* TARGET_KERNEL_ADDITIONAL_CONFIG has been nuked since it's superseed by this new logic
* kernelsavedefconfig will only use the base defconfig

Change-Id: I479c762a9235ed0ef6fcdc79b53e084d5e2d78a6