kernel.bbclass: set HOSTLDFLAGS in KCONFIG_CONFIG_COMMAND

kernel.bbclass: set HOSTLDFLAGS in KCONFIG_CONFIG_COMMAND

Kernel v4.14 and newer contain the following in their Makefile:

HOST_LFS_LDFLAGS := $(shell getconf LFS_LDFLAGS)
HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS)

This breaks our menuconfig, because it can no longer find ncurses if its
not on the host machine. This can be seen in linux-yocto-dev, for
example:

[clsulliv@clsulliv build]$ bitbake virtual/kernel -c menuconfig

  GEN     ./Makefile
  HOSTLD  scripts/kconfig/mconf
/home/clsulliv/yocto/poky/build/tmp/hosttools/ld: cannot find -lncurses
/home/clsulliv/yocto/poky/build/tmp/hosttools/ld: cannot find -ltinfo
collect2: error: ld returned 1 exit status
make[3]: *** [scripts/Makefile.host:99: scripts/kconfig/mconf] Error 1
make[2]: *** [/home/clsulliv/yocto/poky/build/tmp/work-shared/intel-corei7-64/kernel-source/Makefile:504: menuconfig] Error 2
make[1]: *** [Makefile:146: sub-make] Error 2
make: *** [Makefile:24: __sub-make] Error 2
Command failed.
Press any key to continue...

Fix this by setting HOSTLDFLAGS to ${BUILD_LDFLAGS} in our
'make menuconfig' command.

Signed-off-by: California Sullivan <california.l.sullivan@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>