Change the default build dir to 'build'

[screamerbg]

This PR changes the default build dir from .build to build.

Also adds '.mbedignore' to ensure that contents of build are not scanned/included by the build system and reflects the change in the documentation.

Addressing #334 #206

@jaustin @MarceloSalazar @sg- @theotherjimmy

[bridadan]

@screamerbg Has this been tested when building tests? The test building process includes files from the build directory as source files.

[bridadan]

Also, I will need to update mbed-os CI will need to be updated to use the build directory now and not .build. I will look into this.

[screamerbg]

@bridadan That's why I'm raising these with you :)
CC @jupe

[sg-]

this needs to be BUILD

[theotherjimmy]

Any particular reason for the capitalization @sg-?

[jupe]

CC: @miklis @marhil01 : this probably affected to our ci as well..

[sg-]

based on upcoming code base and API guidelines, any directory that is treated as magic will be capitalized.

[screamerbg]

Agree with @sg-. Now changed to 'BUILD'

[sg-]

LGTM 👍

[screamerbg]

@bridadan @jupe @miklis @marhil01 Can you confirm that the CIs are ready for this change?

[bridadan]

@screamerbg I've pinned the CI to use the current mbed-cli version for now (v0.9.5) so this change should not affect the Austin CI. I will migrate the jobs to the newest version of mbed-cli after this has been released.

[screamerbg]

@bridadan Thanks for the response.

@jupe @miklis @marhil01 The new mbed CLI will be released tomorrow. The PR was up for 11 days now so I hope you managed to transition the CI to either use the --build option or bind the CI to specific version of mbed CLI.