iotrace: Finish migrating this to Kconfig

iotrace: Finish migrating this to Kconfig

When I migrated this to Kconfig in commit 68e54040ccc3 ("sandbox: Move
CONFIG_IO_TRACE to Kconfig") I didn't look hard enough for other
details. As explained in the README, this is valid for ARM too. So start
by making this be a prompted question and CMD_IOTRACE depend on IO_TRACE
being enabled.  Next, migrate the information out of README and in to
the appropriate help text for existing options in Kconfig. Finally, make
this option be default y on SANDBOX but not selected as it's valid to
build without it.

Signed-off-by: Tom Rini <trini@konsulko.com>