fs: fix number base behaviour change in fatload/ext*load

Commit 045fa1e1

 "fs: add filesystem switch libary, implement ls and
fsload commands" unified the implementation of fatload and ext*load
with the new command fsload. However, this altered the interpretation
of command-line numbers from always being base-16, to requiring a "0x"
prefix for base-16 numbers. Enhance do_fsload() to allow commands to
specify which base to use.

Use base 0, thus requiring a "0x" prefix for the new fsload command.
This feels much cleaner than assuming base 16.

Use base 16 for the pre-existing fatload and ext*load to prevent a
change in behaviour.

Use base 16 exclusively for the loadaddr environment variable, since
that variable is interpreted in multiple places, so we don't want the
behaviour to change.

Update command help text to make it clear where numbers are assumed to
be hex, and where an explicit "0x" prefix is required.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>