uboot-imx

Makefile: use two double-quotations as a pair

Some editors such as Emacs can highlight source files.
But their parser algorithm is not perfect.

If you use one double-quotation alone, some editor cannot
handle it nicely and mark source lines as a string by mistake.

It is preferable to use two double-quotations as a pair.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>