cmd_elf: add an option for loading ELFs according to PHDRs

The current ELF loading function does a lot of work above and beyond a
simple "loading".  It ignores the real load addresses and loads things
into their virtual (runtime) address.  This is undesirable when we just
want it to load an ELF and let the ELF do the actual C runtime init.

So add a command line option to let people choose to load via either the
program or section headers.  I'd prefer to have program header loading
be the default, but this would break historical behavior, so I'll leave
section header loading as the norm.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>