task-core-boot: Allow DISTRO to disable keymaps via VIRTUAL-RUNTIME_keymaps
keymaps pulls in gettext, which can't build without wide character support.
Rather than have to add a -tiny variant of every MACHINE to drop "keyboard" from
the MACHINE_FEATURES, just enable the DISTRO to define what gets added to
RDEPENDS_task-core-boot when MACHINE_FEATURES includes keyboard via the
VIRTUAL-RUNTIME_keymaps variable. By default, keep "keymaps".
Signed-off-by: Darren Hart <dvhart@linux.intel.com>
