libasm.h: Don't include libebl.h. Define an opaque Ebl handle.

libasm.h: Don't include libebl.h. Define an opaque Ebl handle.

Using libasm isn't really usable without a way to create an Ebl handle.
But we don't support libebl.h (and libebl itself). Just define the
Ebl handle as an opaque struct. Code that uses it needs to figure out
how to instantiate one itself (they cannot in any supportable way...)

Signed-off-by: Mark Wielaard <mark@klomp.org>