subsys/mgmt/hawkbit: change 'Hawkbit' & 'hawkbit' to 'hawkBit'

Change 'Hawkbit' and 'hawkbit' to 'hawkBit' wherever
makes sense, and a bit of touch ups here and there.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>