Libc custom location

Hi Bryan,

This set of patches implements for {e}glibc the same functionality I added for gcc, binutils, gdb, newlib etc, namely the ability to use a CUSTOM {e}glibc, and optionally specify a CUSTOM_LOCATION (if its not default CT_CUSTOM_LOCATION_ROOT_DIR/{e}glibc).

It exists currently on patchworks, but they're pretty out of date vs master;

https://patchwork.ozlabs.org/patch/190790/
https://patchwork.ozlabs.org/patch/190792/
https://patchwork.ozlabs.org/patch/190793/

I didn't pursue these further, but would like to see them applied if possible as its pretty useful feature while developing on a toolchain.

thanks,
David