package/pkg-kconfig: pass HOSTCC during kconfig steps

The kconfig build logic uses the HOSTCC variable to find the host
compiler. It makes sense to explicitly pass a value to this variable,
pointing to the host compiler used by Buildroot.

During the kconfig step, host-ccache is not ready (host-ccache is only
a dependency to the configure step of packages), so we use
$(HOSTCC_NOCCACHE).

Packages currently using the kconfig-package fell into two categories:

 - Those not passing any HOSTCC value. For such packages, it was the
   default host compiler detected by the kconfig build logic that was
   used. ccache was therefore never used. With this commit, those
   packages will now be using the host compiler detected by
   Buildroot. Packages in this situation: at91bootstrap3, barebox,
   busybox, swupdate, uclibc, xvisor.

 - Those passing a HOSTCC value. Such packages were passing $(HOSTCC),
   which doesn't work as host-ccache will not be ready. This commit
   does not fix them, as they still override HOSTCC. It will be fixed
   in followup commits. Packages in this situation: uboot and
   linux. Note that linux was a bit special, because it has a
   KCONFIG_DEPENDENCIES on the toolchain package, so in fact
   host-ccache was ready.

So practically speaking, this commit does not fix anything, as the two
only problematic packages that use $(HOSTCC) are not fixed. However,
it makes things more correct by explicitly telling kconfig which
compiler to use.

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>