x86/build: Don't convert boot/{cmdline,head}.bin back to .S

x86/build: Don't convert boot/{cmdline,head}.bin back to .S

There's no point wasting time converting binaries back to asm source.  Just
use .incbin directly.  Explain in head.S what these binaries are.

Also, explicitly align the blobs.  They contain 4-byte objects, and happen to
be 4-byte aligned currently because of the position of `lret` and the size of
cmdline.S but this is incredibly fragile.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>