setup: move INITRAMFS_COMPRESS under kernel

Move INITRAMFS_COMPRESS under kernel.
pkg_cleanup: simplify a test

Simplify a test.
setup: delete BUILD_KERNEL

Delete BUILD_KERNEL.
setup: move download and list to separate files

Move download and list to separate files.
setup: delete redundant exports

Delete redundant exports.
setup: auto-detect tar decompressor

Auto-detect tar decompressor.