[Buildroot] External toolchain improvements

[Lionel Landwerlin]

Hi Thomas,

I'm interested by your patches, I'm using a STLinux external toolchain
(Linux for SH4 SetTop Boxes).

The problem is that copying only libraries is not enough to build even a
simple program like this one :

int main (void) { return 0; }

With my current toolchain, I also need :

      * crt1.o crti.o crtn.o
      * All headers from the libc & linux

After that, I also have problems with packages using pkg-config.
Pkg-config returns paths relatives to the build directory ( -L/usr/lib
for example). This tries to link with