expose DBus interface property names as symbols

Enable a way to access the property names of a given DBus interface as a
constexpr symbol via the header [1]

4 use-cases:

- printing error / debug l

expose DBus interface property names as symbols

Enable a way to access the property names of a given DBus interface as a
constexpr symbol via the header [1]

4 use-cases:

- printing error / debug logs with the property name.
e.g. '${property} missing from configuration'

- accessing DBus properties in applications which do not yet use the PDI
generated bindings.

- preventing typos in DBus property names

- estimating the impact of removing a given DBus property from an
interface. When using these symbols, it would cause a build failure in
applications relying on the existence of that property.

Tested: a newly added unit test is using this feature.

References:
[1] https://discordapp.com/channels/775381525260664832/867820390406422538/1423317550732673206

Change-Id: I7b2906b6b1b445c3a49868ff1d50ced6ccaa5336
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>

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