at91boostrap makefile, correct typo in line continuation character.

The rule to copy the resulting binary file into the binary directory had
a typo in the line continuation character that results in a spurious error
message being produced.

Signed-off-by: Rob Alley <Rob.Alley@navmanwireless.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>