Exporter does not validate if target supports mbed-OS version

[deepikabhavnani]

Description

• Type: Bug
• Priority: Minor

Compiling NRF51822 gives expected error with mbed OS 5:

mbed compile -m NRF51822 -t GCC_ARM
Could not compile for NRF51822: Target does not support mbed OS 5

Compiling same source code with export and build on uVision is successful.

Expected behavior
Export should throw error for incompatible target and mbed-os combination.

Target
NRF5182

[ciarmcom]

ARM Internal Ref: IOTMORF-1181

[theotherjimmy]

#6512