I've extracted the Chromium-EC encryption functions, they are convenient for signing / verifying firmware on other platforms. Chromium-ec is nice for example code like this:

https://github.com/jhallen/rsa-verify

On the other hand, if you are looking for some generic embedded system code all in C, here is our library (it's been cleaned up for ATSAM and STM32 targets, but we've used in on many other platforms):

https://github.com/nklabs/libnklabs

I think it's most unique feature is the embedded schema-based database- so you can save things like calibration and configuration information in local flash memory (think protocol buffers, but for tiny systems). Recently I've been adding device drivers for all common devices I can find on break-out boards from the Arduino and Raspberry-PI communities.