build: define RTC_SUPPORT as a bool

build: define RTC_SUPPORT as a bool

Currently, RTC_SUPPORT is defined as a tristate, with 'depends on m',
which is supposed to only let it be set to 'm' or 'n'.  However,
scripts/target-metadata.pl will 'select' it, or setting it to 'y', which
defeats it's 'depends on m' restriction.  The users of the symbol are
not expecting it to be necessarily 'm' either, so we can safely use it
as bool.

Newer versions of Linux 'conf' will issue a warning when it detects such
unmet dependencies, and will set it to 'n' instead of 'y', as the
current version does.  In all cases, 'm' is never used.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>