Revert "opkg: disable the use of vfork for the host build"

This reverts commit 02e3c718.

Currently the vfork() code path in opkg is broken and relies on unsupported
ftello() / fseeko() operations on pipes - we need to restructure the code
before we can reconsider this approach.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>