Have you looked into what trezor crypto does? They are using a secp256k library on a stm32f2. So there are embedded libraries that do this especially from the hardware wallet side.
Discussion
I have looked into some I thing uBitcoin but I wasn't happy with the arch. I suppose I could make it pluggable during build time, but libsecp256k1 has my heart in design. If someone else has a worthy dependency I will consider it so thanks for the referalls!
My intentions were to fork libsecp256k1, because we want to add some CPU intrinsic helpers or assembly routines where necessary, and it's one of the most forkable libs I've worked with so thats the motivation.