makechrootpkg: -I to handle multiple packages

Since commit cb3a6ce1, running makechroot 2 times to insert a package in a build
directory require to find a directory without PKGBUILD

cd /var/empty
makechrootpkg -cu -I virtualbox-host-dkms-*-i686.pkg.tar.xz -r <dir>
makechrootpkg -I virtualbox-host-dkms-*-i686.pkg.tar.xz -r <dir>
cd -
makechrootpkg -n -r <dir>

This patch allow makechrootpkg to handle more than one package to be installed
before the build is run and simplify the previous case in

makechrootpkg -ncu -I virtualbox-host-dkms-*-i686.pkg.tar.xz -I virtualbox-guest-dkms-*-i686.pkg.tar.xz -r <dir>

Signed-off-by: Sébastien Luttringer <seblu@seblu.net>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>