Fix ccache handling

When ccache support is enabled and 'make silentoldconfig' is being
started from the following rule:

$(BUILD_DIR)/buildroot-config/auto.conf: $(CONFIG_DIR)/.config
	$(MAKE) $(EXTRAMAKEARGS) HOSTCC="$(HOSTCC_NOCCACHE)" HOSTCXX="$(HOSTCXX_NOCCACHE)" silentoldconfig

then, the Makefile in package/config inherits from HOSTCC/HOSTCXX
values with the ccache prefix. However, if we start from a cleaned
build (after make clean), ccache is not compiled yet, so things break.

To solve this, we pass a noccache variant of HOSTCC/HOSTCXX down to
the package/config Makefile.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>