boot/uboot: use HOSTCC_NOCCACHE as kconfig HOSTCC

At kconfig time, dependencies are not built, and therefore host-ccache
is not ready. Due to this, using $(HOSTCC) as the host compiler in
KCONFIG_OPTS does not work: a "make uboot-menuconfig" invocation from
a clean tree with ccache enabled fails.

This commit fixes this by using $(HOSTCC_NOCCACHE). We cannot rely on
the default value of HOSTCC passed by the kconfig-package
infrastructure, because $(UBOOT_MAKE_OPTS) also contains a HOSTCC
definition that would override the one passed by the kconfig-package
infrastructure.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>