fastboot: change default sparse limit to disabled

Change the default sparse limit to disabled to prevent resparsing files
unless the bootloader or user specifies a sparse limit.  Some
bootloaders issue an erase before every flash command, which causes
earlier parts of the downloaded image to get erased.

Change-Id: Iac6f3b05580aba8b82ed0f2f800979a49c33c691
1 file changed