The UBI ec_hdr has an image_seq field. During attaching UBI expects
that its value is the same for all eraseblocks. The value should be
changed with every ubiformat and is used to detect half written images.

In barebox we use a pseudo random number generated with rand() for this
value. The ubiformat command calls srand(get_time_ns()) to initialize
the pseudo random numbber generator. This is done in the option parser,
so when ubiformat() is called directly (from fastboot for example) the
pseudo random number generator is not initialized and we get the same
values after every barebox restart.

This patch moves the pseudo random number generator initialization
to the place where the numbers are generated. Also use random32()
rather than rand() to generate 32bit values rather than 15bit values
(0 - RAND_MAX).

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>