README: update bootlog for 3.12 on BeagleBone Black

Signed-off-by: Marta Rybczynska <rybczynska@gmail.com>
README: Update installation instructions

Modules need to be compiled, CDC ACM USB gadget is working fine.

Signed-off-by: Marta Rybczynska <rybczynska@gmail.com>
Merge pull request #73 from mrybczyn/3.12

3.12: updated bootlog and installation status