[image] Support archive image formats independently of "imgextract" command

[image] Support archive image formats independently of "imgextract" command

Support for the zlib and gzip archive image formats is currently
included only if the IMAGE_ARCHIVE_CMD is used to enable the
"imgextract" command.

The ability to transparently execute a single-member archive image
without using the "imgextract" command renders this unintuitive: a
user wanting to gain the ability to boot a gzip-compressed kernel
image would expect to have to enable IMAGE_GZIP rather than
IMAGE_ARCHIVE_CMD.

Reverse the inclusion logic, so that archive image formats must now be
enabled explicitly (via IMAGE_GZIP and/or IMAGE_ZLIB), with the
archive image management commands dragged in as needed if any archive
image formats are enabled.  The archive image management commands may
be explicitly disabled via IMAGE_ARCHIVE_CMD if necessary.

This matches the behaviour of IBMGMT_CMD and similar options, where
the relevant commands are included only when something else already
drags in the underlying feature.

Signed-off-by: Michael Brown <mcb30@ipxe.org>