add missing data partition size

this fixes the bug
userdata.img is not generated by build system

Signed-off-by: Alin Jerpelea <alin.jerpelea@sonymobile.com>
README.md: update build instructions

Signed-off-by: Alin Jerpelea <alin.jerpelea@sonymobile.com>
bcmdhd: use sony fw for now

Signed-off-by: Alin Jerpelea <alin.jerpelea@sonymobile.com>