external toolchain: add $(HOST_DIR)/bin to TARGET_PATH

Since the move of many host binaries to $(HOST_DIR), the TARGET_PATH
for internal toolchain had been changed to include $(HOST_DIR)/bin and
$(HOST_DIR)/usr/bin, but this change was not reflected to the
TARGET_PATH used in external toolchain configuration. This patches
fixes this.

We also remove $(STAGING_DIR)/bin and $(STAGING_DIR)/usr/bin from the
TARGET_PATH in external toolchain configuration, since the STAGING_DIR
is not supposed to contain host binaries.

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