add ldflags to cross-do script

This is the reason why I was not able to cross compile for Beaglebone Black (BBB), not sure how I missed the LDFLAGS.

Build log: https://gist.github.com/nandub/9202387
EDIT: Cross compiling from ubuntu saucy to BBB using arm-linux-gnueabihf-gcc (crosstool-NG linaro-1.13.1-4.8-2013.10 - Linaro GCC 2013.10) 4.8.2 20131014 (prerelease)

Command used:
```
CROSS_COMPILE=arm-linux-gnueabihf- CROSS_CFLAGS="-march=armv7-a -mtune=cortex-a8 -marm -mfloat-abi=hard -mfpu=neon -g" CROSS_LDFLAGS="-march=armv7-a -mtune=cortex-a8 -marm -mfloat-abi=hard -mfpu=neon -g" ./cross-do
```