Remove common.h

Additionally kill the global variable: reason, stage; move them to a
separate BootState class instead. Vendor specific recovery code will
need to call getters from Device() class to access these variables.

Bug: 137705917
Test: unit tests pass, boot sailfish into recovery, code search and no
code includes common.h in vendor specific recovery.
Change-Id: Ia50a5ea951212c25548562f29cc9cf78505b5e34