toolchain-external: Introduce kernel headers sanitization

The Nios-II Sourcery external toolchain (the only Nios-II we currently
support) exports broken kernel headers. In particular, these kernels should
be exported using the "headers_install" rule which applies a set of fixes
on the kernel headers so they are suitable for userspace usage.

In order to fix this, add a post-install hook to perform the header fixes
ourselves. The result is equivalent to apply the "headers_install" rule.

Fixes:
http://autobuild.buildroot.net/results/c32/c32ad4bac5f651502e551f7733f702afaa0e742a/



Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>