[nrf noup] boards: nordic: Skip offsets in merged slot

In the merged slot approach, the memory reservation for the MCUboot
header is done in CMake, as it is not obvious, which image includes the
(merged) MCUboot header.
This change will remove the unnecessary gap between images.

Ref: NCSDK-35612

Signed-off-by: Tomasz Chyrowicz <[email protected]>
(cherry picked from commit 077c913)

Author: tomchy

boards/nordic/nrf54h20dk/Kconfig.defconfig

@@ -13,6 +13,7 @@ config MAX_THREAD_BYTES
 	default 3 if USERSPACE
 
 config ROM_START_OFFSET
+	default 0 if NCS_MCUBOOT_BOOTLOADER_SIGN_MERGED_BINARY
 	default 0x800 if BOOTLOADER_MCUBOOT
 
 if !USE_DT_CODE_PARTITION
@@ -39,6 +40,7 @@ config MAX_THREAD_BYTES
 	default 3 if USERSPACE
 
 config ROM_START_OFFSET
+	default 0 if NCS_MCUBOOT_BOOTLOADER_SIGN_MERGED_BINARY
 	default 0x800 if BOOTLOADER_MCUBOOT
 
 endif # BOARD_NRF54H20DK_NRF54H20_CPURAD