cli: Port upstream Busybox hush to U-Boot

cli: Port upstream Busybox hush to U-Boot

Adds new file cli_hush_upstream.c, it is a copy of Busybox hush file as it was of
time to commit 37460f5da.
This commit modifies Busybox hush to not compile some part specific to Busybox
and adds some code needed by U-Boot.
The modifications consists mainly on adding code #if(n)def guards.

For the moment, this refurbished flavor of hush only permits running command
without any keywords (i.e., if and for are not recognized) or variable expansion
(i.e., echo $foo prints foo and not value stored in variable foo).

A new file was also added to define some functions specific to U-Boot.

Signed-off-by: Francis Laniel <francis.laniel@amarulasolutions.com>
Signed-off-by: Harald Seiler <hws@denx.de>