mkarchiso: do not show subdirectory sizes in netboot mode

Only the total size is relevant.

Related to #148.
9 jobs for more-quiet in 67 minutes and 51 seconds (queued for 6 seconds)
latest
Status Name Job ID Coverage
  Check
passed check #34405

00:00:15

 
  Build
passed build_long: [baseline iso] #34408
fast-single-thread

00:18:05

passed build_long: [baseline netboot] #34409
fast-single-thread

00:18:01

passed build_long: [releng iso] #34438
fast-single-thread

00:34:58

passed build_long: [releng netboot] #34411
fast-single-thread

00:32:35

passed build_short: [baseline bootstrap] #34406

00:12:42

passed build_short: [releng bootstrap] #34407

00:13:19

failed build_long: [releng iso] #34410
fast-single-thread

00:02:57

canceled build_long: [releng iso] #34413
fast-single-thread

00:55:15