cmd: set filesize variable in ubi read

After a successful read of a UBI volume, the variable filesize is set
to the number of read bytes. To boot linux with a raw initramfs/initrd,
you must specify the exact size of the initramfs/initrd image in boot
command. If the initramfs/inird is read from UBI volume, $filesize is
required to access the number of read bytes.

Example:
  ubi read ${loadaddr} kernelvol
  ubi read ${fdtaddr} dtbvol
  ubi read ${initrd_addr} initrdvol
  bootz ${loadaddr} ${initrd_addr}:${filesize} ${fdt_addr}

Signed-off-by: Holger Dengler <dengler@linutronix.de>
Reviewed-by: Heiko Schocher <hs@denx.de>