support/gnuconfig: update README.buildroot with reality
4 out of 5 packages who are not using autotools but needed their
gnuconfig files updated were not complying with the recommandation in
support/gnuconfig/README.buildroot. The fifth package was converted to
be like the others: use UPDATE_CONFIG_HOOK as a <pkg>_POST_PATCH_HOOKS
rather than calling the CONFIG_UPDATE macro directly.
Now that all packages are consistent, update the README.buildroot file
to match the reality.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>