build: add facilities for interpreting boolean values

This is another small addition to the build system utlities to make it
easier to determine the truthiness of an arbitrary value.

This change adds the `bool` function, which takes a value and determines
whether the value is "truthy". We consider a value to be truthy if it is
NOT one of: "0", "n", "no", "f" or "false" (all case-insensitive).

If the value is truthy then it is returned as-is. Otherwise, no value
is returned.

Change-Id: I19347f4c3ae00a6b448514a28cc2d9d06f683f25
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed