v2: Do not exit from all scripts but only the current one

This fixes the case:

barebox:/ cat /test
if [ 0 = 0 ]; then
	exit 1
fi
barebox:/ /test
barebox:/ echo $?
0
barebox:/

Also, remove code to not allow exit from main shell. The for(;;) loop
in common/startup.c will bring us back anyway.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>