build_image: Allow disabling custom inode count calculation

This allows us to skip custom inode count calculation by setting
BOARD_*IMAGE_EXTFS_INODE_COUNT to -1, this will end up letting
mke2fs calculate appropriate inode count on its own.

While build_image only allocates exact number of needed inodes
plus 4% spare with .2% margin, mke2fs will allocate as many
inodes as it thinks is appropriate given the filesystem size.

Change-Id: If03d5edae8378be3b305346176067b01163f6f3d