base: Improve handling of switching virtual/x providers

base: Improve handling of switching virtual/x providers

If you build virtual/kernel, then change PREFERRED_PROVIDER_virtual/kernel from say
"linux-yocto" to "linux-yocto-dev", you see errors from the sysroot about overlapping
files. The automatic uninstall logic doesn't trigger since the other recipes is
still technically parsed/buildable.

What we can do is look at the value of PREFERRED_PROVIDER_virtual/X and raise SkipRecipe
(skip parsing) if it provides this thing and its not selected. We skip cases no preferred
provider is set, or the value is in MULTI_PROVIDER_WHITELIST.We also inform the user
if they try to build something which conflicts with the configuration:

$ bitbake linux-yocto-tiny
ERROR: Nothing PROVIDES 'linux-yocto-tiny'
ERROR: linux-yocto-tiny was skipped: PREFERRED_PROVIDER_virtual/kernel set to linux-yocto, not linux-yocto-tiny

[YOCTO #4102]

(From OE-Core rev: 9a3b992b7339f7aa892f1dff7d159747630b045d)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>