ldlinux: Move cmdline processing out of cli.c

ldlinux: Move cmdline processing out of cli.c

While it makes perfect sense for cli.c to read input from the user and
parse certain characters (Ctrl-R, TABs, etc), once that processing is
done it should be up to the caller to decide what to do with the
expanded cmdline string. For example, the logic to figure out what
type of kernel we're loading (calculated from a kernel's filename
extension as typed on the command-line) is currently in cli.c, and
that is just wrong. That kind of logic belongs in ldlinux.c

So move process_command() from cli.c to ldlinux.c and extend it so
that if the user types a label name at the prompt, we load the kernel
for that label as described in the config file.

Now that we've moved all cli history code into edit_cmdline() it seems
like a good time to turn it on permanently.

Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>