luci.mk: make SUBMENU package define customizable and optional

When including luci.mk in external repos it's sometimes usefull to not use
the default LuCI-submenu hierarchy.
This change defines the LUCI_SUBMENU_FORCED variable which completely overrides
the default submenu of the LuCI config-section. When LUCI_SUBMENU_FORCED is not
defined, the default submenu derrived from LUCI_TYPE or "Application" fallback
is used.
Defining LUCI_SUBMENU_FORCED in the package Makefile will just use this value.
Setting it to "none" will not define a submenu at all.
Together with LUCI_SECTION and LUCI_CATEGORY menu items can now created at any
place in the menu structure.

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>