nRF5x: pass ram linker start/length from config system

[andrewleech]

Description

The ram start and length settings can already be defined in targets.json with mbed_ram_start and mbed_ram_size etc.

These are supposed to be used in nrf5x targets to specify in the linker how much ram is set aside for the softdevice. Unfortunately the linker files have these values defined in directly so they can't be overridden by the target settings.
This PR fixes the problem by simply checking if the defines are already set before setting them in the linker scripts.

There was also a minor bug in the python config handler where these boot-loader target settings in were being overridden by a later part of the config system with None. This is also fixed.

Pull request type

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

[theotherjimmy]

Thanks for fixing that config system bug. Nice diff!

[theotherjimmy]

Just for tools changes.

[TacoGrandeTX]

Looks good.

[TacoGrandeTX]

@andrewleech In light of #7959 could you recommend how this should be merged?

[andrewleech]

I've using both of these MR's together in my working branch, they're quite complimentary.

This MR allows the project to set where the start address should be, the other one ensures that the correct start address is passed to the softdevice.

I did consider making the other one work from this config system define rather than the linker element, but decided to leave it as the linker symbol as it's a smaller change to the existing nordic code and it should be safer, the symbol used is just in nordic linker scripts so is less likely to be accidentally changed in a breaking way than the config system symbol which is used for all targets.

[0xc0170]

This needs a rebase, will be scheduled for CI afterwards

[andrewleech]

@0xc0170 I've rebased, should be good to go.

The travis failure above looks pretty unrelated:

The command "W: Failed to fetch http://www.apache.org/dist/cassandra/debian/dists/39x/InRelease Could not connect to www.apache.org:80 (95.216.24.32), connection timed out [IP: 95.216.24.32 80]" failed. Retrying, 2 of 3.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3175
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7957/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2773
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7957/

[mbed-ci]

Test : SUCCESS

Build number : 2980
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7957/2980

[cmonr]

/morph mbed2-build