samples: dfu: usb_mcumgr: fix nRF54LM20 Dongle configuration

Remove mcumgr_usb_cdc_acm instance defined by the board as the sample
already defines its own. Also, move USB configuration from single_init
to usb_mcumgr to reduce configuration duplication, and remove no
longer needed files from the former.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>

Author: Damian-Nordic

samples/dfu/fw_loader/usb_mcumgr/boards/nrf54lm20dongle_nrf54lm20b_cpuapp.conf

@@ -0,0 +1,10 @@
+#
+# Copyright (c) 2026 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+#
+CONFIG_CDC_ACM_SERIAL_VID=0x1915
+CONFIG_CDC_ACM_SERIAL_PID=0x541A
+CONFIG_CDC_ACM_SERIAL_MANUFACTURER_STRING="Nordic Semiconductor ASA"
+CONFIG_CDC_ACM_SERIAL_PRODUCT_STRING="nRF54LM20 Dongle Bootloader"
+CONFIG_MCUMGR_GRP_ZBASIC_STORAGE_ERASE=n

samples/dfu/fw_loader/usb_mcumgr/boards/nrf54lm20dongle_nrf54lm20b_cpuapp.overlay

@@ -10,16 +10,20 @@
 		/delete-property/ zephyr,shell-uart;
 		/delete-property/ zephyr,bt-mon-uart;
 		/delete-property/ zephyr,bt-c2h-uart;
-		zephyr,uart-mcumgr = &cdc_acm_uart0;
 	};
 };
 
 &zephyr_udc0 {
 	/delete-node/ board_cdc_acm_uart;
+	/delete-node/ mcumgr_cdc_acm_uart;
 };
 
 #include "../app.overlay"
 
+&cdc_acm_uart0 {
+	label = "nRF54LM20 Dongle MCUmgr";
+};
+
 /* This is nRF54lm20dongle_nrf54lm20b_cpuapp-specific partitioning for MCUboot and firmware loader.
  * slot0_partition defines RRAM region which can be updated by the firmware loader and booted by
  * MCUboot.

samples/dfu/single_slot/sysbuild/mcuboot/boards/nrf54lm20dongle_nrf54lm20b_cpuapp.conf

@@ -6,7 +6,8 @@
 CONFIG_SYS_CLOCK_EXISTS=n
 CONFIG_NRF_GRTC_TIMER=n
 CONFIG_CRACEN_IKG=n
-CONFIG_FPROTECT=y
+CONFIG_FPROTECT=n
+CONFIG_NCS_MCUBOOT_DISABLE_SELF_RWX=y
 
 # Disable run-time CRACEN provisioning of TRNG data.
 # KMU slots 248, 249 shall be pre-provisioned while programming with secret TRNG values instead.