Arm backend: Make setup.sh handle unset EULA variable (#9446)

If setup.sh was called without --i-agree-to-the-contained-eula or with
ARM_FVP_INSTALL_I_AGREE_TO_THE_CONTAINED_EULA unset all you would get
back is the error message:

"examples/arm/setup.sh: line 88:
ARM_FVP_INSTALL_I_AGREE_TO_THE_CONTAINED_EULA: unbound variable"

This is not a good error message. This commit adds handling of this
variable being unset and gives a proper error message on how to accept
the EULA.