[cmdline] Show commands in alphabetical order

[cmdline] Show commands in alphabetical order

Commands were originally ordered by functional group (e.g. keeping the
image management commands together), with arrays used to impose a
functionally meaningful order within the group.

As the number of commands and functional groups has expanded over the
years, this has become essentially useless as an organising principle.
Switch to sorting commands alphabetically (using the linker table
mechanism).

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