cmd: exit: Fix return value propagation out of environment scripts

Make sure the 'exit' command as well as 'exit $val' command exits
from environment scripts immediately and propagates return value
out of those scripts fully. That means the following behavior is
expected:

"
=> setenv foo 'echo bar ; exit 1' ; run foo ; echo $?
bar
1
=> setenv foo 'echo bar ; exit 0' ; run foo ; echo $?
bar
0
=> setenv foo 'echo bar ; exit -2' ; run foo ; echo $?
bar
0
"

As well as the followin behavior:

"
=> setenv foo 'echo bar ; exit 3 ; echo fail'; run foo; echo $?
bar
3
=> setenv foo 'echo bar ; exit 1 ; echo fail'; run foo; echo $?
bar
1
=> setenv foo 'echo bar ; exit 0 ; echo fail'; run foo; echo $?
bar
0
=> setenv foo 'echo bar ; exit -1 ; echo fail'; run foo; echo $?
bar
0
=> setenv foo 'echo bar ; exit -2 ; echo fail'; run foo; echo $?
bar
0
=> setenv foo 'echo bar ; exit ; echo fail'; run foo; echo $?
bar
0
"

Fixes: 8c4e3b79bd ("cmd: exit: Fix return value")
Reviewed-by: Hector Palacios <hector.palacios@digi.com>
Signed-off-by: Marek Vasut <marex@denx.de>

doc/usage/cmd/exit.rst

@@ -37,4 +37,6 @@ executed.
 Return value
 ------------
 
-$? is always set to 0 (true).
+$? is default set to 0 (true). In case zero or positive integer parameter
+is passed to the command, the return value is the parameter value. In case
+negative integer parameter is passed to the command, the return value is 0.