[nrf fromlist] boards: nordic: nrf54h20dk iron board ROM offset

Provide proper ROM offset for the nRF54H20 iron board
variant when using MCUBoot as a bootloader. This is
needed to ensure that application init vector is correctly
set up.

Upstream PR #: 88298

Signed-off-by: Michal Kozikowski <[email protected]>
(cherry picked from commit 4cf4c1ec6dbeeea8f6889fb819fc6cc610dbae12)

Author: nordic-mik7

boards/nordic/nrf54h20dk/Kconfig.defconfig

@@ -17,3 +17,10 @@ config MAX_THREAD_BYTES
 	default 3 if USERSPACE
 
 endif # BOARD_NRF54H20DK_NRF54H20_CPURAD
+
+if BOARD_NRF54H20DK_NRF54H20_CPUAPP_IRON
+
+config ROM_START_OFFSET
+	default 0x800 if BOOTLOADER_MCUBOOT
+
+endif # BOARD_NRF54H20DK_NRF54H20_CPUAPP_IRON