[GRUB][SYSDATA] Pass sysdata boot slot to bootscript env

When booting from the live medium (i.e. USB) the bootscript is called with the configfile command. That "erases" the default evnrionment and the variable set by sysdata during switchslot status check is lost.

This commits forces the value read by sysdata to be passed to the environment used by the bootscript.

Signed-off-by: Nicola Sparnacci nicola.sparnacci@seco.com