[sb2] Fixed a typo

Signed-off-by: Juha Kallioinen <juha.kallioinen@jollamobile.com>
[sb2] Exit if target root is not accessible

Signed-off-by: Juha Kallioinen <juha.kallioinen@jollamobile.com>
[sb2] Better clean up after sb2 error

If sb2 script exits with an error the sb2 session directory under /tmp
is now deleted unless the new option -N is given.

Signed-off-by: Juha Kallioinen <juha.kallioinen@jollamobile.com>
Merge pull request #2 from kaltsi/master

Some housekeeping patches