build: implement make check and make package/X/check

This is intended to be used for a wide array of package sanity checks.

The first check that is implemented is for the hash of downloaded files.
It checks:
  - Missing hash
  - Use of SHA256 instead of MD5
  - dl/<file> hash not matching hash in makefile
  - deprecated MD5SUM variable

The deprecated MD5SUM variable check is skipped for feeds/ until OpenWrt
is updated as well

Signed-off-by: Felix Fietkau <nbd@nbd.name>