recovery: Drop the "--stages" / '-g' argument.

This was introduced in commit c87bab10.
But the stage info should be passed through BCB only (there's a
dedicated field in struct bootloader_message).

This CL removes it from recovery arguments, and also moves 'stage'
variable to std::string.

Test: 'stage' variable is not used by any device-specific recovery code.
Test: Code search shows no hit of '--stages' use.
Change-Id: Iccbde578a13255f2b55dd4a928e9ecf487f16b97