buildsteps: allow oe-init-build-env to be changed

Some distributions don't use oe-init-build-env but have their own environment
script.  Add a env_cmd property to RunPreamble to specify the command
(defaulting to oe-init-build-env) and if it works, set it as a property for
other steps to use.

Signed-off-by: Ross Burton <ross.burton@intel.com>