toolchain/crosstool-NG: fix bundled config files

Bundled config files have architecture-specific values in them,
which may break if buildroot is configured with another architecture
that does not forcibly set these option.

For example, the bundled config files are for x86_64, and define:
    CT_ARCH_TUNE="generic"

This comes from the BR2_GCC_TARGET_TUNE config option (in buildroot) that
is set accordingly to the selected (arch,sub-arch).

But if someone configures buildroot for, say, generic ARM, then the
BR2_GCC_TARGET_TUNE config option is not set, and the crosstool-NG backend
Makefile believes it should not be pushed down to the crosstool-NG config
file.

BUT...

The crosstool-NG backend Makefile can not forcibly push BR2_GCC_TARGET_TUNE
down to the CT-NG config file. If BR2_GCC_TARGET_TUNE is empty, the user can
still set CT_ARCH_TUNE by running ctng-menuconfig.

The backend Makefile already passes such values only if they are set.

In the end, we can't push options as-is to the crosstool-NG config, but we
MUST provide sane bundled config files, which this patch does.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>