Create "direct" install sub command that will be called in a posttrans on package mode installations

As part of https://fedoraproject.org/wiki/Changes/BootLoaderUpdatesPhase1, for package mode installations, we will update the bootloader (copy from /usr to boot or ESP) as part of the posttrans scriptlet.

This is a slightly different path as the current update path where we have the systemd-run logic, and we notably don't have any prepared metadata.

We can start by creating a command that does a simple copy (like the scripts do) and then iterate on merging the logic between the package mode and the image mode use case.

See change and current sample/basic installation scripts:

• https://fedoraproject.org/wiki/Changes/BootLoaderUpdatesPhase1
• https://src.fedoraproject.org/rpms/grub2/pull-request/136
• https://src.fedoraproject.org/rpms/shim/pull-request/4

See related issue: https://github.com/coreos/bootupd/issues/926