dumplogcat:  add the ability to pull correct last_kmsg

Some devices use a differnt file for last__kmsg.  This will allow
the pull of a different file.
To use define BOARD_USES_ALT_KMSG_LOCATION := "path to last_kmsg"
inside your board config file.

Change-Id: I30e3bc3220712bb7b47d6d2f9dc23e3babf9c4a2