ti816x: Modernize the defconfig

- Switch to using <configs/ti_armv7_omap.h> and family.  This lets us
  drop lots of custom defines.
- Ensure that our default environment uses DEFAULT_LINUX_BOOT_ENV so
  that Linux will boot correctly.
- Enable CONFIG_DISTRO_DEFAULTS
- Switch to using CONFIG_OF_CONTROL
- Various other cleanups to match other SoCs in the family line.

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