-u- in Akk. (alongside with -i-) and Arm. (Syr. and Mnd.) is likely a secondary development under the influence of b-.
For the meaning ‘lap’, see *barak- ‘chest, thorax’. See discussion in [Holma 1911 95] where the author tends to treat birku ‘knee’ and birku ‘genitals’ as the same word, quoting, at the same time, Arb. rakab- ‘pubis’ as a possible cognate to the latter.
Formed after the active participle pattern; strangely enough, usually not quoted in comparative studies (see [DRS], [LGz]). Cf. also brk ‘s’agenouiller’ [BK 1 116]. As for bark-, birk- ‘genou’ quoted in [GD 153], it is not clear whether it is Daṯ. or Mhr.