kernel-build: fix kernel_menuconfig breakage by forcing YACC

kernel-build: fix kernel_menuconfig breakage by forcing YACC

Commit 965f341aa9fd ("build: fix host menu config targets using
ncurses") has moved host's path with pkg-config (usually /usr/bin) at
the first place in PATH variable, which is now causing issues with bison
as BISON_PKGDATADIR points into STAGING_DIR_HOST, but the actual bison
used is the one under host PATH (usually /usr/bin/bison), leading to the
following strange failures:

 $ make target/linux/clean kernel_menuconfig V=sc

 export MAKEFLAGS= ;make -C /somewhere/linux-4.19.81 menuconfig
 make -f ./scripts/Makefile.build obj=scripts/kconfig menuconfig
  ...
  bison -oscripts/kconfig/zconf.tab.c -t -l scripts/kconfig/zconf.y
  staging_dir/host/bin/m4: cannot open `staging_dir/host/share/bison/bison.m4': No such file or directory
  staging_dir/host/bin/m4: cannot open `staging_dir/host/share/bison/c-skel.m4': No such file or directory
  ...
  gcc -Wp,-MD,scripts/kconfig/.zconf.tab.o.d <...snip...> -o scripts/kconfig/zconf.tab.o scripts/kconfig/zconf.tab.c
  gcc: error: scripts/kconfig/zconf.tab.c: No such file or directory
  gcc: fatal error: no input files

Fix this by forcing usage of bison under STAGING_DIR_HOST/bin via YACC
make variable.

Cc: Thomas Albers <thomas.gameiro@gmail.com>
Cc: Stijn Tintel <stijn@linux-ipv6.be>
Cc: Eneas U de Queiroz <cotequeiroz@gmail.com>
Tested-by: Ivan Revyakin <LovingFox@GMail.com>
Tested-by: Thomas Albers <thomas.gameiro@googlemail.com>
Ref: https://forum.openwrt.org/t/bpi-r64-kernel-4-19-kernel-menuconfig-error
Fixes: 965f341aa9fd ("build: fix host menu config targets using ncurses")
Signed-off-by: Petr Štetiar <ynezz@true.cz>