board: .gitignore: ignore board-specific generated files

Ignore
  - board/cray/L1/bootscript.{c|image}
  - board/matrix_vision/mvblm7/bootscript.img
  - board/maxtir_vision/mvsmr/bootscript.img

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Andre Schwarz <andre.schwarz@matrix-vision.de>

board/cray/L1/.gitignore

+bootscript.c
+bootscript.image

board/matrix_vision/mvblm7/.gitignore

+bootscript.img

board/matrix_vision/mvsmr/.gitignore

+bootscript.img