manual: add notes about depending on a virtual package

Of course, do not forget to add the proper build and runtime dependencies for

this package!

Notes on depending on a virtual package

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When adding a package that requires a certain +FEATURE+ provided by a virtual

package, you have to use +depends on BR2_PACKAGE_HAS_FEATURE+, like so:

---------------------------

config BR2_PACKAGE_HAS_FEATURE

    bool

config BR2_PACKAGE_FOO

    bool "foo"

    depends on BR2_PACKAGE_HAS_FEATURE

---------------------------

Notes on depending on a specific provider

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If your package really requires a specific provider, then you'll have to

make your package +depends on+ this provider; you can _not_ +select+ a

provider.

Let's take an example with two providers for a +FEATURE+:

---------------------------

config BR2_PACKAGE_HAS_FEATURE

    bool

config BR2_PACKAGE_FOO

    bool "foo"

    select BR2_PACKAGE_HAS_FEATURE

config BR2_PACKAGE_BAR

    bool "bar"

    select BR2_PACKAGE_HAS_FEATURE

---------------------------

And you are adding a package that needs +FEATURE+ as provided by +foo+,

but not as provided by +bar+.

If you were to use +select BR2_PACKAGE_FOO+, then the user would still

be able to select +BR2_PACKAGE_BAR+ in the menuconfig. This would create

a configuration inconsistency, whereby two providers of the same +FEATURE+

would be enabled at once, one explicitly set by the user, the other

implicitly by your +select+.

Instead, you have to use +depends on BR2_PACKAGE_FOO+, which avoids any

implicit configuration inconsistency.