Gives more fexibility to kernel installation

[salvadore]

Summary

Introduces a new USEKERNCONFNAME, that works as follows:

• if USEKERNCONFNAME is undefined, then nothing is changed;
• if USEKERNCONFNAME is defined, then install all kernels in a directory named exactly as the kernel configuration.

For example, if KERNCONF = GENERIC CURRENT STABLE, then the 3 kernels would be installed in /boot/GENERIC, /boot/CURRENT, /boot/STABLE respectively. No /boot/kernel would be created.
Then, /boot/loader.conf should be updated with the new default kernel name (by setting the "kernel" variable) or a /boot/kernel file should be created manually (by copying a valid kernel directory, by soft linking to a valid kernel directory...).

Test Plan

I tested successfully the patch. I did not test what happens if no /boot/kernel file exists and the kernel variable in /boot/loader.conf is not defined.

[salvadore]

Rename USEKERNNAME to USEKERNCONFNAME and reverts logic: now if USEKERNCONFNAME is defined then directory will be called as kernel configurations, otherwise everything works as usual.

[salvadore]

Fix style: before I used ifndef, but it seems that if !defined is prefered.

[salvadore]

The patch is too old and does not not work as expected (or as I remember).

I am going to fix it anyway for personal use. If anyone is interested in it, please let me know: in that case I would reopen the review with the updated patch once it is ready.