Instead of making support code depend on variants or platforms, the
latter should select what they need explicitely.

Otherwise this starts looking weird when support code depends on
!XTENSA_PLATFORM_FOO && !XTENSA_PLATFORM_BAR etc.

This also includes some minor fixlets like converting bool and default
to def_bool and fixing indentation and whitespace errors.
Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>