Cellular: Allow cellular modules to override network registration mode

[AriParkkila]

Description

Cellular network registration is by default in mode 2, that is defined in 3GPP TS 27.007 as [,<[lac>,]<[ci>],[],[]]

That value must be configurable because some modems support only certain modes on specific technologies and also in some cases it's required to support extended modes, such as requested in the issue #7496

Pull request type

[ ] Fix
[x] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[AriParkkila]

@mirelachirica please review

[AnttiKauppila]

"#define" for numbers 0 and 2 would help reading for people unfamiliar with Cellular

[AriParkkila]

I did not use define because the value comes from 3GPP spec and you need to read that in any case to understand what you are doing. Also when the mode is a define and a modem specific porting requires a new mode they also need to make a define also in the generic codes at AT layer. But I can change that if you think its more clear with define?

[cmonr]

I'd like to echo the sentiment of no magic numbers where possible, even if they are hardcoded in some spec.

[AnttiKauppila]

#define works in this case as a hint, it might be the case that developer has read the spec and with this hint s/he finds the context immediately without reading the spec again. Let's define those.

[AriParkkila]

@AnttiKauppila please review

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2927
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7860/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2547
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7860/

[mbed-ci]

Test : SUCCESS

Build number : 2678
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7860/2678