mcuboot VS libopencm3

I've been tinkering with MCUboot. It works w/ Zephyr, Mynewt, NuttX, RIOT, Mbed OS ...

Reset vector to main(): mcuboot is the recommended bootloader. You configure your flash partition map so that the reset vector points to the boot_partition start. E.g. for an STM32F476ZG-Nucleo board, I did

I was using the STM32F4 family and ran into the problem of uboot seeming too heavy. The project wasn't using freertos so there was some extra porting that needed to be done but the documentation over at (https://github.com/mcu-tools/mcuboot) helped out a lot.

Some of the vendor IDEs have the option to generate makefiles for a project. Depending on the processor, you could also opt out to a different system. A lot of the popular cortex-m stuff is supported by libopencm3.

You can also look at examples of HALs such as - libopencm3 (Though this is Cortex only) - RIOT OS (though this requires an RTOS)

Yeah by the looks of it I am going to have to expand my project do creating a portable driver interface as well. Abstracting hardware interfaces to create portable code is fairly common in embedded development. For example, libopencm3 does this and has great support for STM32 parts but very limited support for SAM parts.

TinyUSB or Libopencm3 are probably your best options (although check they support the SAME70).

that is fair. SPI isn't very portable across Arm cpu vendors because all peripherals are vendor specific, i.e. the registers will differ a lot and APIs reflect that. Finding common ground is hard unless using specifically designed portable libraries like https://github.com/libopencm3/libopencm3