km/common: simplify debug environment

The debug environment which is stored in textfiles in the
scripts directory was reworked. Two usecase are now present
which can be executed simply from the default environment:
run develop: this configures the environment to setup the
             rootfs via nfs
run ramfs: this configures the environment to setup the
           rootfs in ram

Each architecture now has a "arch" variable which is used
to load the architecture specific debug scripts and to set
the rootpath for NFS.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>