libopkg: fix md5sum calculation

libopkg: fix md5sum calculation

This regression prevents MD5 checksums from being checked. Packages are
still installed, but this raises several issues:

- if only MD5 checksums are provided in the package list, it is trivial
  for an attacker to modify the content of a package, since checksum
  verification is bypassed. If both MD5 and SHA256 checksums are
  provided, then SHA256 is correctly verified and the attack is not
  possible.

- future efforts to harden checksum verification would prevent package
  installation.

Note that OpenWrt has switched to SHA256 for all its packages several
years ago. As a result, this bug does not affect OpenWrt packages from the
official package feeds.

However, custom package repositories that only use MD5 are affected.

Initially submitted at https://github.com/openwrt/openwrt/pull/3087

Fixes: 33f7b80aa325 ("libopkg: drop custom md5 implementation,
unconditionally enable sha256 support")

Signed-off-by: Alexander Ryzhov <github@ryzhov-al.ru>
[Add commit description]
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>