galileo: Check for errors after downloading newlib tarball
galileo: Check for errors after downloading newlib tarball

Repeat the file existence and MD5 checks in build_newlib.sh after the
newlib tarball is downloaded and exit if either of the checks fail.

MohamedSeliem/contiki,arurke/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki