phase1: rework dlprune Step

[f00b4r0]

Note to self, currently fails with:

find dl/ -mindepth 1 -atime +15 -delete -print
 in dir /builder/master_bcm4908_generic/build (timeout 1200 secs)
 watching logfiles {}
 argv: b'find dl/ -mindepth 1 -atime +15 -delete -print'
 using PTY: False
find: cannot delete ‘dl/ath10k-ct-firmware-2020-11-08’: Directory not empty
program finished with exit code 1
elapsedTime=0.012581

Furthermore the -delete -print order is wrong: deleted files aren't printed.