toolchain/buildroot: properly handle SSP

The current SSP handling is incomplete.

First we need to build uClibc with SSP support for a complete
"experience".

Second, it doesn't hurt to add -fstack-protector-all to the
CFLAGS/CXXFLAGS since most users would expect buildroot to do this
rather than adding the flags themselves.

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