Add TARGET_OTA_ENABLED for targets that support OTA DFU
[NUCLEO_F302R8,F334R8,L053R8] Increase ADC sampling time.

This change allows to pass the MBED_A8 test.
Merge pull request #520 from bcostm/dev_analogin

[HAL] NUCLEO_F302R8,F334R8,L053R8 - Increase ADC sampling time.