entity-manager: Add meson option to cache config

For the development use-case configuration is often changed and the
general expectation is for a service to re-parse it's configuration when
it is re

entity-manager: Add meson option to cache config

For the development use-case configuration is often changed and the
general expectation is for a service to re-parse it's configuration when
it is restarted.

Add a meson option which controls if the configuration (usually in
/var/configuration/system.json) is cached there, or not cached at all.

The new meson option is 'new-device-detection' and by
default it is true, so there is no behavior change when it is not
configured.

Tested: on Tyan S5549 board.
The configuration is cached when the option is left as default and the
configuration is not written to cache when it is set to false.

```
Jan 21 15:50:22 s5549-bmc-a0423f63b4a5 systemd[1]: Starting Entity Manager...
Jan 21 15:50:22 s5549-bmc-a0423f63b4a5 systemd[1]: Started Entity Manager.
Jan 21 15:50:35 s5549-bmc-a0423f63b4a5 entity-manager[32126]: Inventory Added: Supermicro PWS 920P SQ 1
Jan 21 15:50:35 s5549-bmc-a0423f63b4a5 entity-manager[32126]: Inventory Added: MBX 1.60 Chassis
Jan 21 15:50:35 s5549-bmc-a0423f63b4a5 entity-manager[32126]: Inventory Added: Tyan S5549 Baseboard
```

The cache directory now only contains the 'version' file which is
separate from the caching concern, and not affected by this patch.

```
ls -l /var/configuration/
-rw-r--r-- 1 root root 10 Jan 16 11:50 version
```

Change-Id: I584d87c549d3115da04b464702c7fda8ee7720ed
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>

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entity-manager: move logDeviceAdded/Removed

functions do not need to be 'inline' and since they are normal
functions, they can go into a .cpp file to be compiled separately.

Functions otherwise unc

entity-manager: move logDeviceAdded/Removed

functions do not need to be 'inline' and since they are normal
functions, they can go into a .cpp file to be compiled separately.

Functions otherwise unchanged.

Tested: Inspection only.

References:
[1] https://en.cppreference.com/w/cpp/language/inline.html

Change-Id: I194d86f17f90d54d6be2286a52482457435f4c36
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>

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