Automatically define a SYSROOT variable

[tpetazzoni]

If libraries that don't have a .pc file usable by pkg-config are used, it is often necessary to make references to the toolchain sysroot in some build properties of Eclipse. It would therefore be nice if the Buildroot plugin could automatically define a SYSROOT variable, whose contents is equal to the output of the CROSS-gcc -print-sysroot command.