toolchain-buildroot: mark eglibc as deprecated

eglibc is a dead project and has not been making any release since a
while, now that glibc is back and kicking. So let's deprecated our
eglibc support.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>