configs: buildroot: Reduce fd limit to accelerate build

When the limit of open fd is very high, fakeroot used by buildroot takes
forever to start: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=920913



For example my default docker configuration has a file limit of
1073741816, which is unnecessary and causes the build to hang for 4
minutes. Drop the file limit before making buildroot.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>