Generic, RPi, Allwinner: disable initramfs compression, use lz4 for Allwinner

[kszaq]

Quoting kernel config (https://github.com/torvalds/linux/blob/v5.0/usr/Kconfig#L137-L143):

  Do not compress the built-in initramfs at all. This may sound wasteful
  in space, but, you should be aware that the built-in initramfs will be
  compressed at a later stage anyways along with the rest of the kernel,
  on those architectures that support this. However, not compressing the
  initramfs may lead to slightly higher memory consumption during a
  short time at boot, while both the cpio image and the unpacked
  filesystem image will be present in memory simultaneously

We are using bzip2 for Generic and gzip for RPi kernel compression, there is no point to additionally enable initramfs compression.

[kszaq]

Thanks to @jernejsk added:

• Allwinner/aarch64: compress initramfs with lz4
• Allwinner/arm: use uncompressed initramfs, compress kernel image with lz4

[jernejsk]

Can you please add lz4:host dependency to kernel package?

[MilhouseVH]

RPi/RPi2/Generic all now drop CONFIG_DECOMPRESS_GZIP=y after this PR - not sure how we missed that! Will update the confs in #3434

DECOMPRESS_GZIP is being selected by:

config RD_GZIP
        bool "Support initial ramdisk/ramfs compressed using gzip"
        depends on BLK_DEV_INITRD
        default y
        select DECOMPRESS_GZIP
        help
          Support loading of a gzip encoded initial ramdisk or cpio buffer.
          If unsure, say Y.