boot: Create a common BOOT_DEFAULTS for distro and bootstd

These two features use a lot of common options. Move them into a common
CONFIG to reduce duplication.

Use 'select' for most options since these are things that boards aren't
supposed to override. For now it is not possible to disable
BOOT_DEFAULTS but we may take another look later.

Note that five options use 'imply' to match existing behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Rework a bit so we don't grow so many platforms unintentionally]
Signed-off-by: Tom Rini <trini@konsulko.com>