mboot: fix cmdline; a few more layout tweaks

mboot: fix cmdline; a few more layout tweaks

Fix module command lines (it was overwriting the main kernel command
line); a few minor layout tweaks.  In particular, we require the
section header to be page-aligned, but not the subsequent sections.

With this, I can get Xen to boot.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>