Use SMP kernel for Alpha (udeb) builds

Dear all,

As per [1] and our recent discussions the generic 4.x kernels seem to no 
longer work on Alpha machines which also renders any installer images 
using the generic 4.x kernels non-working.

[1]: https://lists.debian.org/debian-alpha/2017/03/msg00007.html

Confirmed on:
* AlphaStation 200 (w/EV4 x 1)
* AlphaStation 255 (w/EV45 x 1)
* Personal Workstation 500au (w/EV56 x 1)
* AlphaServer DS20E (w/EV67 x 2)

Also expected on:
* AXPpci33 (w/LCA4 x 1)
* AlphaStation 500 (w/EV56 x 1)
* AlphaServer DS25 (w/EV68CB x 2)
* AlphaServer ES45 (w/EV68CB x 4)

The following two patches should switch the used kernels to the SMP 
version. As:

(1) I don't exactly know how to build images using multiple kernels 
(i.e. what happens if $TEMP_KERNEL has multiple kernel names in it, 
which seems to be supported according to [2], will the image creation in 
e.g. [3] than run multiple times automatically?) and I don't want to 
break things,

[2]: 
https://salsa.debian.org/installer-team/debian-installer/blob/master/build/config/dir#L79

[3]: 
https://salsa.debian.org/installer-team/debian-installer/blob/master/build/config/alpha/netboot.cfg

(2) I can't find a similar example for another architecture and

(3) the images with the generic kernels are non-working anyhow,

...I just omitted the generic ones for now.

This is sort of a workaround and does not fix the actual problem which 
is yet unknown, but I believe getting working installer images is more 
important at the moment. With working installer images more people could 
get involved and maybe sometime in the future someone has enough time 
and effort to invest in fixing the actual problem.

## Patches ##

1. 
https://salsa.debian.org/frank-scheiner-guest/linux/commit/865cacfd7722b346629082ab3094b6ad93964095

2. 
https://salsa.debian.org/frank-scheiner-guest/debian-installer/commit/7269679bec8bae997ef5ed7619e9f8df2e184134

I think both patches are already enough to produce the needed alpha-smp 
udebs and will allow to produce working installer images (e.g. netboot 
images might work instantly and could be an alternative way for Bob to 
reinstall his PWS).

What do you think? Is there anything obvious missing?

Cheers,
Frank