cli: hush: Adjust 'run' command to run each line of the env var

The run command treats each argument an an environment variable. It gets the
value of each variable and executes it as a command. If an environment
variable contains a newline and the hush cli is used, it is supposed to
execute each line one after the other.

Normally a newline signals to hush to exit - this is used in normal command
line entry - after a command is entered we want to return to allow the user
to enter the next one. But environment variables obviously need to execute
to completion.

Add a special case for the execution of environment variables which
continues when a newline is seen, and add a few tests to check this
behaviour.

Note: it's not impossible that this may cause regressions in other areas.
I can't think of a case but with any change of behaviour with limited test
coverage there is always a risk. From what I can tell this behaviour has
been around since at least U-Boot 2011.03, although this pre-dates sandbox
and I have not tested it on real hardware.

Reported-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>