[image] Fail "imgexec"/"boot" if the image to execute is ambiguous
If there is more than one loaded image, refuse to automatically select
the image to execute. There are at least two possible cases, with
different "correct" answers:
1. User loads image A by mistake, then loads image B and types "boot".
User wants to execute image B.
2. User loads image A, then loads image B (which patches image A), then
types "boot". User wants to execute image A.
If a user actually wants to load multiple images, they must explicitly
specify which image is to be executed.
We can just treat all non-kernel images as initrds, which matches our
behaviour for multiboot kernels. This allows us to eliminate initrd as
an image type, and treat the "initrd" command as just another synonym for
"imgfetch".