fix(firmware): restore omi/sysbuild/mcuboot.conf as regular file (#6350) (#6845)

Commit 32ccac5 intended to replace a symlink with a regular
Kconfig file, but git saved the new content as the symlink's target
string, producing a broken symlink. ls reports mode lrwxr-xr-x and
the "target" is the 717-byte Kconfig body, so the path is
unresolvable and the MCUboot build fails with
'undefined reference to __device_dts_ord_149'.

Replace the broken symlink with a regular file holding the same
content. The body is byte-identical to
omi/firmware/bootloader/mcuboot/mcuboot.conf, so this matches what a
correct symlink would have resolved to.

Fixes #6350

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

Author: mvanhorn

omi/firmware/omi/sysbuild/mcuboot.conf

@@ -0,0 +1,27 @@
+# Example of sample specific Kconfig changes when building sample with MCUboot
+# when using sysbuild.
+CONFIG_SPI_NOR=y
+CONFIG_MCUBOOT_LOG_LEVEL_WRN=y
+CONFIG_BOOT_UPGRADE_ONLY=y
+CONFIG_MCUBOOT_DOWNGRADE_PREVENTION=y
+CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y
+
+CONFIG_NORDIC_QSPI_NOR=n
+CONFIG_BOOT_MAX_IMG_SECTORS=256
+CONFIG_GPIO=y
+CONFIG_SPI=y
+CONFIG_FLASH=y
+CONFIG_FLASH_JESD216_API=y
+CONFIG_SPI_NOR_SFDP_DEVICETREE=y
+CONFIG_SPI_NOR_FLASH_LAYOUT_PAGE_SIZE=4096
+CONFIG_MULTITHREADING=y
+CONFIG_DISK_DRIVER_SDMMC=y
+CONFIG_DISK_ACCESS=y
+CONFIG_DISK_DRIVER_SDMMC=y
+
+CONFIG_FLASH_LOG_LEVEL_DBG=n
+
+CONFIG_NRF53_MULTI_IMAGE_UPDATE=y
+CONFIG_BOOT_UPGRADE_ONLY=y
+CONFIG_BOOT_MAX_IMG_SECTORS=256
+CONFIG_MCUBOOT_DOWNGRADE_PREVENTION=y