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