Release v0.8.0

New:

• .lib do not allow named branches (only rev hashes) #224
• mbed publish --message <msg> to allow mbed publish automation #228
• Support multiple locations to requirements.txt (was deprecated in mbed-os ARMmbed/mbed-os#409)
• Add -N/--artifact-name option to compile to match tools/build.py and tools/test.py options #229
• Report 12 SHA revision hashes in output. Use full 40 SHA hash in .lib
• Bump minor version to 0.8

Fixes:

• Update toolchain settings to use MBED_ prefix (related to ARMmbed/mbed-os#2033)
• Deprecate mbed compile --tests in favour of mbed test --compile and general mbed test workflows #225