WIP: Use config fragments for kernel configuration

Adds support for config fragments - small diffs which define what kernel config options must be disabled/enabled.

Use _kconfig_name to define the base defconfig. Note: "_config" must be removed from APKBUILD.

Diff must be named "_kconfig_name.arch"

Example: for base defconfig sprd_sharkl3_defconfig and arch aarch64 the file name will be sprd_sharkl3_defconfig.aarch64

The file needs to be created manually (e.g. with touch), it can be empty.

Related: #1824 , pmaports!1812