EFM32: make mbed_rtx.h depend of families instead of targets

[amq]

Description

Currently, custom targets based on these families would require a fork of mbed-os, otherwise the build would fail on an undefined INITIAL_SP:

• EFM32GG
• EFM32HG
• EFM32LG
• EFM32PG
• EFM32WG

This is due to specific targets like EFM32GG_STK3700 being used in mbed_rtx.h, instead of families such as EFM32GG.

The newest families EFR32MG12 and EFM32PG12 do not have this issue. This PR aligns mbed_rtx.h with the approach of these new families.

Pull request type

[ ] Fix
[X] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[stevew817]

LGTM

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2160
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6991/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 1950
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6991/1950

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1786
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6991/