Disallow building of mbed OS 2 + RTOS, and remove mbed OS 5 support for Cortext-A

[theotherjimmy]

Description

I found my prior implementation of disabling Cortex-A unsatisfactory.
This PR actually fixes a long-standing bug: you can compile mbed OS 5 for mbed OS 2 only targets.

Todos

• /morph test

[sg-]

@theotherjimmy from travis, https://travis-ci.org/ARMmbed/mbed-os/builds/238031052#L1806

[TomoYamanaka]

@theotherjimmy

This PR seems that the below build error message is missed.
https://github.com/ARMmbed/mbed-os/pull/4415/files#diff-10c649f2fb1884b9dadbf45a6fe74e84L129

• RZ_A1H Will be supported in mbed OS 5.6.
• To use the RZ_A1H, please checkout the mbed OS 5.4 release branch.
• See (LINK snip) for more information

Will you plan on reflecting these error messages after the test pass?

[theotherjimmy]

@sg- This should pass travis

@TomoYamanaka The message is now in.

[theotherjimmy]

/morph export-build

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph export-build

Output

mbed Build Number: 47

All exports and builds passed!

[theotherjimmy]

@sg- do we need a / morph test now?

[studavekar]

/morph test

[mbed-bot]

Result: FAILURE

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 410

Build Prep failed!

[studavekar]

Failure because of Jenkins git plugin

Receiving objects: 62% (78356/125416), 108.26 MiB | 39.00 KiB/s
21:02:44 at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:1799)
21:02:44 at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:1525)
21:02:44 at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.access$300(CliGitAPIImpl.java:65)

re-triggering
/morph test

[TomoYamanaka]

@theotherjimmy

@TomoYamanaka The message is now in.

Thank you for contacting.
I confirmed the build error message. LGTM.

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 412

All builds and test passed!