Re: Bootloaders other than u-boot?

[Gregory Nutt]

Bottom line: has anyone used any bootloader other than u-boot to allow usb dfu 
or RNDIS-type firmware updates, and that easily boot NuttX?

There are a few very tiny, minimal function bootloaders that I have used 
with SAMA5's in the past:

   ./sama5d3x-ek/src/nor_main.c
   ./sama5d4-ek/src/at25_main.c
   ./sama5d4-ek/src/dram_main.c

These are probably too primitive to meet your needs.  They have the 
advantage only in that they are for the SAMA5 and are based on NuttX so 
they are easy to modify or extend.