Optimize build system

Add support to run individual commands like clean/build/package
with build-stack.sh script.

Generally all target does build and doesn't clean the project. Cleaning
all the components and building them again takes lot of time so don't
clean the components with all target, while running all target if there
is an error for the single component, running all again will start fresh
build and makes debugging take unnecessary long time, fresh checkout are
anyway clean so should be fine to build without cleaning.

This will also be useful to package everything without taking long time
when single component is modified, in such case, just clean the single
component and run the all target on all the components.

If full cleaning is required then clean target can be run with build-stack.sh
script.

Also, don't run build command by default when no command is specified.

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>