ext2load: increase read speed

This patch dramatically drops the amount of time u-boot needs to read a
file from an ext2 partition.  On a typical 2 to 5 MB file (kernels and
initrds) it goes from tens of seconds to a couple seconds.

All we are doing here is grouping contiguous blocks into one read.

Boot tested on Globalscale Technologies Dreamplug (Kirkwood ARM SoC)
with three different files.  sha1sums were calculated in Linux
userspace, and then confirmed after ext2load.

Signed-off-by: Jason Cooper <u-boot@lakedaemon.net>
Tested-by: Eric Nelson <eric.nelson@boundarydevices.com>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>