configs: add 'TC_PATH' to specify default toolchain path

Every instance of repo directory needs a copy of toolchain taking up
large disk space.
Now we can have single toolchain directory for multiple repo instances.
To use, define TC_PATH environment variable as the toolchain location.

Signed-off-by: Arnold Gabriel Benedict <ArnoldGabriel.Benedict@arm.com>
Change-Id: I70838a7e53676766327c8e8f0ab285af69df1132