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>