fastboot: Make tags_offset a command line option.

This matches what mkbootimg supports in 5d4af6314fded027952941110a520dc9aa234a14

It will allow "fastboot flash:raw ..." to work for devices that need tags_offset.

Change-Id: I4ceca528f88cc8ede5742f8a67bec972e41eebe7
1 file changed