mkarchiso: do not require setting boot modes when building only netboot artifacts

netboot does not need any of the boot loaders or their files.

Fixes #150.
8 jobs for netboot-no-bootmodes in 63 minutes and 12 seconds (queued for 6 seconds)
latest
Status Name Job ID Coverage
  Check
passed check #34422

00:00:15

 
  Build
passed build_long: [baseline iso] #34425
fast-single-thread

00:29:46

passed build_long: [baseline netboot] #34426
fast-single-thread

00:17:10

passed build_long: [releng iso] #34427
fast-single-thread

00:55:20

passed build_long: [releng netboot] #34428
fast-single-thread

00:32:03

passed build_short: [baseline bootstrap] #34423

00:20:50

passed build_short: [releng bootstrap] #34439

00:13:38

failed build_short: [releng bootstrap] #34424

00:02:27