cmake: use GLOBAL property instead TARGET properties for scoping

Targets are not available in script mode.
To support the Zephyr scoping feature used by snippets and yaml module
then this commit moves from using custom targets to use GLOBAL
properties for scopes.

A scope property is prefixed with `<scope>:<property>` to avoid naming
collisions.
A `scope:<scope-name>` global property is used to track created scopes.
Tracking valid scopes ensure that properties are only set on known
scopes and thus catches typos / naming errors.

Add zephyr_scope_exists() and zephyr_get_scoped() to abstract the
implementation details of the scoped property retrieval and refactor
current code to use them.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Luca Burelli <l.burelli@arduino.cc>