Introduce configuration options to build an "hardened" OpenWRT.

Options to enable Stack-Smashing Protection, FORTIFY_SOURCE and RELRO
have been introduced.

uClibc makefile now automatically detects if SSP support is necessary.

hostapd makefile has been fixed to use "^" as sed separator since
using a comma was problematic when using "-Wl,-z,now" and the like in
TARGET_CFLAGS.

Currently enabling SSP on user space depends on enabling SSP kernel
side, this is due to the fact that TARGET_CFLAGS are used to build
kernel modules (at least). Suggestions on how to avoid this are welcome.
Using "select" instead of "depends on" doesn't seem to work with choice
entries.

Tested with a lantiq (WBMR) router, GCC 4.8, uClibc and a subset of
the available packages.
Needs to be tested with GCC 4.9 and the remaining packages.
PIE not currently included.

Signed-off-by: Alessandro Di Federico <ale+owrt@clearmind.me>

SVN-Revision: 44005