phelum's post in Booting legacy kernel with current U-Boot (OrangePi A20 board) was marked as the answer
To enable U-Boot to boot an old kernel the U-Boot option CONFIG_ARMV7_NONSEC must NOT be selected. I suspect it is selected in the Armbian U-Boot.
There is also the CONFIG_OLD_SUNXI_KERNEL_COMPAT option that apparently slows DRAM access and is sub-optimal for new kernels. I've tested with this option not selected and the OrangePi A20 board here works fine with both old and new kernels booting with this new U-Boot.
phelum's post in Compiling RTL8189es driver was marked as the answer
My slightly less clumsy solution is to modify the Makefile for each of these RTL drivers so the "src" variable is loaded correctly.
The patch line is: export src := $(abspath $(srctree)/$(src))
This line must precede the "EXTRA_CFLAGS += -I$(src)/include" line near the start of each Makefile.