Support managing system in a chroot (bsc#1199840)

[lslezak]

Problem

When running YaST in a container we need to somehow detect that YaST should manage the system mounted in the /mnt and not the container itself.

• Lets use YAST_SCR_TARGET environment variable for that

Solution

• Initialize SCR in the target chroot when starting YaST
• See related https://github.com/yast/yast-yast2/pull/1258

[yast-bot]

:heavy_check_mark: Public Jenkins job #64 successfully finished :heavy_check_mark: Created OBS submit request #978964

[yast-bot]

:heavy_check_mark: Internal Jenkins job #54 successfully finished :heavy_check_mark: Created IBS submit request #276725