configs/nitrogen8m: add u-boot-nodtb.bin custom target

Recent change introduced in [1] use the "nodtb" variant of u-boot
image. This broke the nitrogen8m_defconfig which was not including the
generation of this U-Boot image. This patch add this target.

[1] https://git.buildroot.org/buildroot/commit/?id=d130f0a83700b52ba51ebde22885f0f9e577ee29

Fixes:
- https://gitlab.com/buildroot.org/buildroot/-/jobs/389451762

Signed-off-by: Julien Olivain <juju@cotds.org>
Tested-by: Gary Bisson <bisson.gary@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

configs/nitrogen8m_defconfig

@@ -40,6 +40,8 @@ BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
 BR2_TARGET_UBOOT_CUSTOM_TARBALL=y
 # Latest revision of branch boundary-imx_v2017.03_4.9.51_imx8m_ga
 BR2_TARGET_UBOOT_CUSTOM_TARBALL_LOCATION="https://github.com/boundarydevices/u-boot-imx6/archive/b661d854.tar.gz"
+BR2_TARGET_UBOOT_FORMAT_CUSTOM=y
+BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME="u-boot-nodtb.bin"
 BR2_TARGET_UBOOT_NEEDS_DTC=y
 BR2_TARGET_UBOOT_SPL=y