Create firmware symlinks at build time

See here:

https://github.com/CyanogenMod/android_device_htc_evita/commit/615af286400bc52c7d9167fc7feeaa900fc656dc

https://github.com/CyanogenMod/android_device_htc_s4-common/commit/a035473fff508015f0d9cd06a7ff3260136fab89