pkg: refactor out a pkg_read_fd()

We already have to do this for reading the pubkey, just pull it out for
other uses. The ECC signer will use this to verify the bootstrap if
the PUBKEY mechanism is used.

Reviewed by: bapt, emaste

(cherry picked from commit 2ecfc040a09f8c42f67bbfdcc4bd02ef84dac8b7)