Thanks for the changes to the bootloader, dietpi now boots without problems.
Has anyone tested the wifi module? …It doesn’t work in my Rock4CPlus. 
In Journalctl’s log I have the (brcmfmac) error -2:
ott 29 23:12:20 DietPi systemd[1]: Starting systemd-rfkill.service - Load/Save RF Kill Switch Status…
ott 29 23:12:20 DietPi kernel:brcmfmac: brcmf_c_process_txcap_blob: no txcap_blob available (err=-2)
ott 29 23:12:20 DietPi kernel: brcmfmac: brcmf_c_preinit_dcmds: Firmware: BCM4345/6 wl0: Aug 25 2015 18:58:57 version 7.45.69 (r581703) FWID 01-24037f6e
ott 29 23:12:20 DietPi systemd[1]: Reached target bluetooth.target - Bluetooth Support.
ott 29 23:12:20 DietPi systemd[1]: Finished ifupdown-pre.service - Helper to synchronize boot up for ifupdown.
ott 29 23:12:21 DietPi systemd[1]: Started systemd-rfkill.service - Load/Save RF Kill Switch Status.
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM: chip id 107
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM: features 0x2f
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM4345C0
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM4345C0 (003.001.025) build 0000
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM: firmware Patch file not found, tried:
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM: ‘brcm/BCM4345C0.radxa,rock-4c-plus.hcd’
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM: ‘brcm/BCM4345C0.hcd’
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM: ‘brcm/BCM.radxa,rock-4c-plus.hcd’
ott 29 23:12:21 DietPi kernel: Bluetooth: hci0: BCM: ‘brcm/BCM.hcd’
ott 29 23:12:21 DietPi systemd[1]: Expecting device sys-subsystem-net-devices-wlan0.device - /sys/subsystem/net/devices/wlan0