generate-config.pl: Be more verbose about missing options

For instance, the current 3.10 kernel build fails at the end with:

unused option: BRCMFMAC_PCIE
unused option: FW_LOADER_USER_HELPER_FALLBACK
unused option: KEXEC_FILE
unused option: RANDOMIZE_BASE

However, it's not obvious that only the _last_ one is actually fatal to
the build. After this change it's at least somewhat better:

warning: unused option: BRCMFMAC_PCIE
warning: unused option: FW_LOADER_USER_HELPER_FALLBACK
warning: unused option: KEXEC_FILE
error: unused option: RANDOMIZE_BASE