bitbake.conf: change inode calculation for genext2fs [BUGFIX#138]
genext2fs 1.4.1 creates minimal inode number based on specified rootfs
directory. This is desired in some embedded devices as storage/memory
are precious. However it's not suitable in development phase where target
rootfs may be changed heavily on the fly (remote debug, test, ...).
Sometimes this may even cause "No space on device" error due to limited
free inodes exhaulted at the 1st boot.
Here a new option is added to allow falling back to original 1.3 behavior,
i.e caculating inode number based on specified bytes-per-inode parameter.
