build-appliance-image: fix git proxy access

Updated the name of git proxy access script.

Fixes [YOCTO #4161]

Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
rpm: Ensure rpm depends on rpm-postinsts

If package-management isn't in IMAGE_FEATURES, the postinstall handler
wasn't being installed. rpm needs to depend on this to ensure it does
get installed.

[YOCTO #4160]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
remake: Fix out of tree builds

remake fails with errors during configure due to the out of tree build changes.
This ensures the configure commands run correctly on files in ${S}.

[YOCTO #4139]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>