[tools/build_api] Retrun truthy value on success from build_*

[theotherjimmy]

before:

$  python tools/build.py  -m NRF51_DK --source . -t GCC_ARM,ARM --build build --silent
[SNIP]
Completed in: (3.39)s

Build skipped:
  * GCC_ARM::NRF51_DK
  * ARM::NRF51_DK

after:

$  python tools/build.py  -m NRF51_DK --source . -t GCC_ARM,ARM --build build --silent
[SNIP]
Completed in: (3.43)s

Build successes:
  * GCC_ARM::NRF51_DK
  * ARM::NRF51_DK

[theotherjimmy]

cc @bridadan @screamerbg

[bridadan]

+1 from me.

The only concern I have is about the change to the build_lib function since that's legacy code. I would defer to @screamerbg on that one.

[screamerbg]

LGTM. Don't think something will break.