Default (and possibly architecture dependents) HAL modules go here.
libhardware.so eventually should contain *just* the HAL hub
(hardware.c), everything in it should be rewritten as modules.
Modules are .so in /system/libs/hw/ and have a well defined naming
convention:
/system/libs/hw/<*_HARDWARE_MODULE_ID>.<ro.product.board>.so
/system/libs/hw/<*_HARDWARE_MODULE_ID>.<ro.board.platform>.so
/system/libs/hw/<*_HARDWARE_MODULE_ID>.<ro.arch>.so
/system/libs/hw/<*_HARDWARE_MODULE_ID>.default.so
They also have a well defined interface which lives in include/hardware/.
A module can have several variants: "default", "arch" and "board", and they're
loaded in the "board", "arch" and "default" order.
The source code for the "board" variant, usually lives under partners/...
The source code for "default" and "arch" would usually
live under hardware/modules/.