Make SSP available for package builds

[areynold]

Partially addresses #22

Already tested by @areynold, @dismantl, and @hawkinswnaf.
Assigning to @technosopher to merge, since it will require a build server toolchain rebuild.

Instructions:

1. Add src-git areynold git://github.com/areynold/commotion-feed.git to feeds.conf. (ssp_test does not need to be merged.)
2. ./setup.sh
3. cd openwrt
4. make menuconfig
   4a. Select Commotion > Utilities > ssp_test, which includes SSP compile flags in its Makefile. (Note: I have occasionally had to deselect and reselect Advanced Configuration Options > Toolchain Options > Enable Stack-Smashing-Protection to make the test utility available.)
   4b. Save and exit
5. make V=s
6. Flash router.
7. Telnet to router. No need to run through setup wizard.
8. cd /root
9. Run ./smash to find the address of the vulnerable function. Note that the address format is 0x followed by 8 digits. You only need the digits after "0x". If smash provides fewer than 8, pad the address with 0s as needed. Also note that you will need to reverse the address on little-endian machines. See examples below.
10. Smash will display a 4-character string. Re-enter the string. Smash should abort with the "stack smashing detected" error.
11. Run smash and enter a 3-character string. Smash should print your string and exit normally.
12. Type printf "meat\x01\x12\x23\x34\x01\x12\x23\x34\x01\x12\x23\x34\x01\x12\x23\x34" | ./smash. If the package was compiled with SSP, smash will abort with the message *** stack smashing detected ***: smash terminated Trace/breakpoint trap. If not, it will execute the target function and print a message to the screen.
13. Run through setup wizard and confirm standard function tests (node meshes, etc.).

Alternately, you can run strings ./smash and grep for "__stack_chk_fail", but that's not nearly as cool.

Examples:
If smash returns 0x804849d, you will first need to pad the address (making it 0804849d).

On a big-endian device (such as ubiquiti routers) you would enter \x08\x04\x84\x9d.
On a little-endian device (such as x86) you would enter \x9d\x84\x04\x08.