Collect ignored directories in scan and use in exporters

[theotherjimmy]

This patch set resolves issue #3919 by unifying the logic for computing
excluded paths into scan_resources. This relieves the exporter from
having to compute this information itself, and significantly speeds up
exporters that had to compute exclusions (GNU ARM Eclipse at this time).

[0xc0170]

Is travis failure related?

Scan: FEATURE_IPV4
FAILED!
    List of features doesn't match
    Expected: 'IPV4,STORAGE,UVISOR'
    Got: 'IPV4,STORAGE,UVISOR'
test25:'test recursive feature collisions'(K64F) Scan: test25
Scan: FEATURE_STORAGE
FAILED!
    Expected key 'exception_msg' was not found in configuration data
test26:'test recursive feature configurations'(test_target) Scan: test26
Scan: FEATURE_IPV4
FAILED!
    Error while getting configuration!
    Attempt to override undefined parameter 'lib2.test' in 'application[*]'
test27:'test removing features'(test_target) Scan: test27
OK
test28:'test uvisor feature'(test_target) Scan: test28
OK
test29:'error when bootloader not found'(K64F) OK

[theotherjimmy]

@0xc0170 Real failure. Should be fixed.

[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: 97

All exports and builds passed!