webos_cmake: Add weak assignments for WEBOS_TARGET_*

* WEBOS_TARGET_MACHINE_IMPL, WEBOS_TARGET_CORE_OS are
  defined only in webos.inc, building for different DISTRO than webos
  fails to parse all recipes which are inheriting webos_cmake

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I0012c83184b29fa59c4fb6855b40a5bcb4a22f24
Reviewed-on: https://g2g.palm.com/1852
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
webos_cmake: Remove dependency on MACHINE variable

* sstate checksums were depending on MACHINE variable even for recipes which
  don't inherit webos_machine_dep/webos_machine_impl_dep/webos_kernel_dep
* when some recipe inherits webos_machine_dep and also native, it was setting
  WEBOS_TARGET_MACHINE which isn't correct, native tools should be MACHINE
  agnostic

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I52d54d72d76ac8bb832a4ca1db9787f284726e10
Reviewed-on: https://g2g.palm.com/1853
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
db8: Remove dependency on luna-prefs

* it builds without luna-prefs
* luna-prefs is now MACHINE_ARCH, so this dependency would force
  MACHINE_ARCH also for db8, activitymanager, configurator, filecache,
  mojomail-*, ...

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I70470f75116c9964e6fe4d754a711a95cf22e01b
Reviewed-on: https://g2g.palm.com/1854
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
webos-connman-adapter: Inherit webos_machine_dep

* it depends on MACHINE_ARCH luna-prefs

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I5fbed1e49199f0748d82628bbb4256f97cfd975f
Reviewed-on: https://g2g.palm.com/1858
Reviewed-by: Build Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>