managerd: add nvmem boot-state backend

New nvmem backend added to boot-state.
This backend supports update rollback mechanism for
TI AM335x's SOC or processor which has RTC scratch registers.
For boot-state mechanism RTC scratch_register2 fields used as below
 - byte zero hold boot count value
 - byte one hold the update available state
 - byte two hold the version
 - byte four hold boot count magic

Signed-off-by: Moses Christopher <BollavarapuMoses.Christopher@in.bosch.com>