Reduce default size of boot partition to 16MB.
Reduce default size of boot partition to 16MB.

The environment is not stored there anymore, so we should keep it
minimal size by default.

I wanted to reduce it to the alignment size, or 8MB, but this didn't
work for some reason, and created a boot partition that was slightly
bigger and not aligned, despite containing no files.

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender