MVBLM7, MVSMR: fix Makefile (cleanup bootscript.img)

Fix MVBLM7 and MVSMR Makefiles for correct out-of-tree building
(create "bootscript.img" in build directory instead of source
directory) and cleanup (remove "bootscript.img" when cleaning up).

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Andre Schwarz <andre.schwarz@matrix-vision.de>