Multilib SDKs differ only in the environment variables set, so nativesdk-meson's
setup script needs to write a cross file for each environment.
Rename the shipped meson.cross to meson.cross.template, as it cannot be used
directly. Now that post-relocate scripts are called once for each environment,
the generated meson.cross can be prefixed with TARGET_PREFIX to ensure it is
unique.
Finally rewrite the setup script to use string.Template to perform the expansion
instead of hand-coding the logic.
Signed-off-by: Ross Burton <ross.burton@intel.com>
