[depends] fix bson Makefile so it can be used without unified depends

[wsnipex]

this allows users to easily install libbson on linux when not using unified depends

You just run: make -C tools/depends/target/libbson PREFIX=/usr/local

[garbear]

awesome! I added the libbson step to the readme: https://github.com/garbear/xbmc/tree/retroplayer-game-library