linux: don't override HOSTCC for kconfig

linux: don't override HOSTCC for kconfig

Kconfig uses either pkg-config or hard-coded /usr/include paths to find
the ncurses or ncursesw library. If ncursesw is found, it will include
<ncursesw.h>. Since Buildroot's host-ncurses doesn't install a .pc file,
and linux.mk anyway doesn't pass the pkg-config options to find the host
pkg-config files, Kconfig will always find the system's ncursesw.h.

However, since commit dde090c299 (linux: fix passing of host CFLAGS and
LDFLAGS) HOST_LDFLAGS is passed to the linux build system. Thus, if
host-ncurses was already built before 'make linux-menuconfig' is called,
the build will pick up libncurses from the host directory, which is NOT
widechar. Thus, two different ncurses configurations are mixed into the
final mconf program. This will result in serious breakage in the
rendering of the menus (lots of @ and question mark characters).

As a workaround (suggested by Yann), don't pass HOST_CFLAGS and
HOST_LDFLAGS when running kconfig commands. For kconfig, we should never
need host packages anyway. This way, the kconfig calls will always use
the system's ncurses and never our host-ncurses.

Note that the same problem could pop up for other kconfig packages as
well if we ever pass HOST_CFLAGS/HOST_LDFLAGS to them. We could force
HOSTCC=$(HOSTCC) directly in kconfig-package. However, for now there
are no other packages that exhibit this problem, so this can be
revisited when they do.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: David De Grave <david.degrave@essensium.com>
Cc: Scott Fan <fancp2007@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>