Fix postmarketos-splash version detection during generation of splash screens

I noticed that when there isn't the chroot_rootfs_device folder and pmbootstrap install all the packages at once, the generation of the splash screens get triggered but the system doesn't have the information of the installed packages yet.

With this addition, if the apk info -v | grep postmarketos-splash command returns an empty string, it use latest version from repository with apk search -x postmarketos-splash (which is the version that is getting installed).

This resolves the useless re-generation of the splash screens.