Libffi-dev package: python pip

Hello! I am new to openwrt.
Raspberry pi 3 b+
source: https://openwrt.org/toh/raspberry_pi_foundation/raspberry_pi

i am trying to do these things inside of openwrt:
pip install service_identity
pip install cryptography
pip install effi

unfortunately, i have come to the conclusion that i need libffi-dev.
libffi-dev is not available in opkg.

this person had the same problem
https://groups.google.com/forum/#!msg/python-cffi/ba-H1GqsUhs/_wDeh66RDAAJ

Question 1: can i use this package?
https://packages.debian.org/stretch/libffi-dev

Question 2: do i need to compile libffi-dev from source, using this link? can someone guide me?
https://sourceware.org/libffi/

thanks.
Oliver