base-files: accept gzipped nand sysupgrade images

base-files: accept gzipped nand sysupgrade images

When firmware images only contained compressed kernels and squashfs roots,
uncompressed tar files were a good option. We are now using UBIFS images,
both raw and tarred, as well as ubinized (full UBI partition) images, all
of which benefit greatly from compression.

For example, a raw ubinized backup taken from a running Askey RT4230W REV6
(such full backups can be restored via the LUCI's sysupgrade UI) is over
400 MB, but compresses to less than 10 MB.

This commit adds support for gzipped versions of all file types already
accepted by the nand sysupgrade mechanism, be them raw or tarred.

Signed-off-by: Rodrigo Balerdi <lanchon@gmail.com>