libopkg: pass PKG_UPGRADE variable to package scripts

When installing or upgrading packages, pass an PKG_UPGRADE environment
variable to maintainer scripts to allow the to perform different actions
depending on whether a package is freshly installed or upgraded.

During installs the PKG_UPGRADE variable will be set to 0 and during
upgrades to 1.

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