toolchain: remove redundant and incorrect --with-build-time-tools option

This option is already part of the gcc configure options through the
BR2_CONFIGURE_BUILD_TOOLS variable (in toolchain/Makefile.in).

Additionnally, the value that was passed in the AVR32 specific case
was incorrect: it was $(STAGING_DIR)/$(REAL_GNU_TARGET_NAME)/bin
instead of $(STAGING_DIR)/usr/$(REAL_GNU_TARGET_NAME)/bin.

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