baremetal target are broken

[MarcTreySonos]

baremetal targets do not have any sysroot, as a consequence no TARGET_SYSROOT exists for those targets.

this change make the presence of TARGET_SYSROOT mandatory:
https://github.com/snipsco/dinghy/blob/dinghy-test/0.4.21/dinghy-build/src/bindgen_macros.rs#L48

this break the compilation on the cortex-m baremetal target

proposed fix: only add the --sysroot when the env var TARGET_SYSROOT exists

[kali]

To be fair, baremetal was never an objective of dinghy. That saif if we can make it work...