memdisk: remove hard-coded assumptions for a specific RM segment

memdisk: remove hard-coded assumptions for a specific RM segment

Remove some hard-coded assumptions about having an RM segment at a
specific location.  This should allow us to relocate the RM code once
the PM code knows where DOS free memory ends.  This in turn allows the
PM code to load the boot sector almost anywhere it wants to go in low
memory, as is required for 100% compliant El Torito support.

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