[EXPORT][CoIDE] problem with missing linker script

- a missing linker script configures the scatter file option (linker
script path in CoIDE) to 'None' not to ''
[EXPORT][CoIDE] [DISCO_407] and [NUCLEO_F4x1] memory areas

The templates now contains the memory areas preconfigured as they are
used in linker scripts. When the CoIDE linker option 'Use memory layout
from memory window' is manually switch on the correct memory layout is
already there.
Add Nucleo targets in MBED_A8 test (those having a DAC)
Remove duplicated entry
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.
[NUCLEO_F072RB] Fix 8-bit data access issue with SPI slave