[ARM] riscpc: fix decompressor font file handling

font_acorn_8x8.o was being built in drivers/video/console/ twice
during a build _in the same location_ - once for the kernel proper,
and once for the decompressor.  The result is when you came to run an
install target, the kernel was always rebuilt due to this file
apparantly having been built with different compiler arguments.

Solve this by making a local copy at build time in the decompressor's
directory.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>