mkbootimg: use int for os_version and os_patch_level

The parse_int function attempts to interpret the string to detect
base, which leads to it assuming strings starting with 0 are base
8. This obviously fails for certain dates, so use int instead.

Bug: 27498078
Bug: 22914603
Change-Id: I50b1fb3d7876f2ec17d00649cc9a2d23af2aec2b