jffs2/mtdparts: Fix problem with usage from JFFS2 and MTDPARTS together

Currently using JFFS2 with MTDPARTS enabled doesn't work. This is because
mtdparts_init() is available in both files, cmd_mtdparts.c and
cmd_jffs2.c. Please note that in the original cmd_jffs2.c file (before
the jffs2/mtdparts command/file split those 2 different versions
already existed. So this is nothing new. The main problem is that the
variables "current_dev" and "current_partnum" are declared in both
files now. This doesn't work.

This patch now changes the names of those variable to more specific
names: "current_mtd_dev" and "current_mtd_partnum". This is because
this patch also changes the declaration from static to global, so
that they can be used from both files.

Please note that my first tests were not successful. The MTD devices
selected via mtdparts are now accessed but I'm failing to see the
directory listed via the "ls" command. Nothing is displayed. Perhaps
I didn't generate the JFFS2 image correctly (I never used JFFS2 in
U-Boot before). Not sure. Perhaps somebody else could take a look at
this as well. I'll continue looking into this on Monday.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Ilya Yanok <yanok@emcraft.com>
Cc: Renaud barbier <renaud.barbier@ge.com>