tools: add uush

add a micro micro interactive shell which allows to just
run commands either in the foreground or in the background
using "&" appended to the command.
Useful for testing purposes, especially running directly
in loader.img before full usr.img build is available.
No cmdline arguments.

"uush" is a module, so you can add it to the image by using
make image=uush

Examples of use:

with just loader.img (AArch64):

./scripts/imgedit.py setargs build/release/loader.img --nomount /tools/uush.so
qemu-system-aarch64 -nographic -M virt -enable-kvm -kernel loader.img -cpu host -m 1024M

with full usr.img and scripts (x64):
./scripts/run.py -c 1 -V -e tools/uush.so

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>