kbuild: be more foregiving on init section naming

In the whitelist function of modpost now use the same
check to identify init_section as in other places of modpost.
This has the effect that we now recognize sections named
.init.text.19 as init sections and we no longer warn
when we see these.

At the same time make surrounding code readable by dropping
use of temporary flags.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>