options COMPAT_AOUT to file UPDATING to know

=== Reason:

In compiling the kernel again after a long time after 'pkg upgrade' the 
following errors. The Intel graphics card is in use and something had 
changed, the 'startx' did not start the XFCE session. This was the 
reason to compile the kernel again with the new sources of today. After 
two retries taking some time to complite, it would be helpful to ...

=== Symptom:

--- kernel.full ---
linking kernel.full
ld.lld: error: undefined symbol: aout_sysvec
 >>> referenced by imgact_gzip.c:240 (/usr/src/sys/kern/imgact_gzip.c:240)
 >>> imgact_gzip.o:(Flush)

=== Resolution:

Adding

options         COMPAT_AOUT

to the kernel configuration file.

This added the necessary 'imgact_aout.o' to the linking and the 
'aout_sysvec' was found.

__________________

It would be nice if a message was written in the /usr/src/UPDATING about 
this to prevent the unnecessary retries. Probably COMPAT_LINUXKPI needs 
this setting with 'device gzip' since COMPAT_AOUT was defined in a 
kernel module object, not in a kernel object file. Somewhere in time the 
COMPAT_AOUT did not exist after the 11-stable and this time it was 
necessary (today 13:th of July 2018) with the Intel driver. It has been 
necessary somewhere between 16:th of May to today (from these kernel 
timestamps). Who knows I don't know.

It would be a good idea to add a note to the UPDATING as information for 
all of the Intel graphics users about the aout to be necessary to use 
less time in testing the error.

br, jla