iflag: Fix dependencies, factor out static components of iflag.h

iflag: Fix dependencies, factor out static components of iflag.h

Multi-dependencies don't work as expected, especially not across Make
versions, this is why we don't use them and read the instructions list
multiple times.

iflag.h has a lot of static content, so factor out the static content.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>