Add kernel configuration bbappend

Add linux-%.bbappend and associated conditional .inc file to supply
demo-related kernel configuration using the new AGL_KCONFIG_FRAGMENTS
variable scheme that is being added in meta-agl-core.  The moved
configuration fragments are copies of the ones from meta-agl-core
except for:
- The configuration from the previously linux-yocto-specific
  hciuart.cfg has been rationalized with btusb.cfg and hid.cfg.
- The actual CAN hardware device support from can-bus.cfg has been
  moved to a new can-devices.cfg here.
- To simplify things, the disable_most.cfg configuration to ensure
  that the upstream kernel MOST driver is disabled to avoid
  conflicting with the externally built AGL version has been made
  generic instead of being specific to the Kingfisher support in
  meta-agl-bsp.  It has also been updated a bit for newer kernels.

Bug-AGL: SPEC-3983

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I32e4da1ca53e9395239b47ab2191a88e3cd39d4c