[coreboot] How to add an offset at the coreboot creation ?

[Jean Francois TRAP]

Hello,
I have a flash memory 16M.
I would like coreboot position in the last 256K of memory. address : 0xFC0000
Here is my current coreboot.rom

Coreboot.rom 16384 kB, bootblocksize 18487, romsize 16777216, offset 0x0
Alignment : 64 bytes

Name                                       Offset               Type                Size
Fallback/romstage                     0x0                   stage                17490
Fallback/coreboot_ram              0x44c0              stage                85656
Config                                      0x193c0            raw                  3682
(empty)                                    0x1a280            null                   16651480

How to add an offset at the coreboot creation ?
thanks for your help

M. TRAP