Take a look at the Intel® Quark™ BSP Build and Software User Guide for instructions on building a custom image.
The source files can be found Galileo Software Downloads, I recommend you to download the "BSP Patches and Build Instructions" to build an image for Galileo.
The source files can also be found at Intel® Download Center by searching for Intel Galileo.
alternatively, if you are interested in the Intel IoT devkit image you can grab from meta-intel-iot-devkit - Intel IoT Developer Kit metadata and bitbake from there. This would give you the multilibc (eglibc + uclibc) image available on https://software.intel.com/en-us/iot/getting-started