toolchain/helpers: add check for mandatory uClibc options

We currently only check that the Buildroot configuration matches what is
available in the toolchain.

Since we're going to remove the check for LFS and make it a mandatory
feature, we will lose the corresponding buildroot option, so we won't be
able to use check_uclibc_feature as-is.

Introduce a magic value passed as the buildroot option name to recognise
checks for mandatory uclibc options that do not have a corresponding
option in buildroot.

If the buildroot option name is empty then the check is against a
mandatory uclibc option.

If a mandatory uclibc option is missing we reject the toolchain as being
unusable by buildroot.

[Thomas: minor tweaks in comment, remove space instead of tab.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>