package/environment-setup: add cmake alias

In order to simplify the usage of a buildroot toolchain within
the buildroot shell, a "cmake" alias is provided to call cmake
with the correct toolchain file and options.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>