kexec: allow relaxed placement specification via command line

kexec: allow relaxed placement specification via command line

Rather than just allowing a fixed address or fully automatic placement,
also allow for specifying an upper bound. Especially on EFI systems,
where firmware memory use is commonly less predictable than on legacy
BIOS ones, this makes success of the reservation more likely when
automatic placement is not an option (e.g. because of special DMA
restrictions of devices involved in actually carrying out the dump).

Also take the opportunity to actually add text to the "crashkernel"
entry in the command line option doc.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>