binutils,gcc: use correct --prefix

The cross binutils and cross gcc are actually going to be executed
from $(STAGING_DIR)/usr, so the correct prefix is $(STAGING_DIR)/usr
and not /usr.

This also fixes what is known as the "AVR32 toolchain build failure",
which was due to the fact that the prefix directory wasn't writable
(since it was /usr).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>