Arm backend: Make setup.sh sourceable to run single functions (#7934)

If setup is run like
`./examples/arm/setup.sh` ..., it works like before.
If it is run like
`source ./examples/arm/setup.sh`, the functions in the script
can be run afterwards, for example you can just run
`setup_toolchain`

To increase modularity, I broke out creating setup_path.sh
to its own function.
This avoids breaking setup_path.sh if setting up the toolchain
and FVP fails/ is aborted.

Signed-off-by: Erik Lundell <erik.lundell@arm.com>