minnowboard_max_defconfig: grub2 needs WCHAR

Since commit 5ffafd23 (grub2: bump up version), grub2 needs a
toolchain with WCHAR support. However, the defconfigs that use grub2
were not updated at the time.

Add BR2_TOOLCHAIN_BUILDROOT_WCHAR=y to minnowboard_max_defconfig.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Erico Nunes <nunes.erico@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Cc: Vincent Stehlé <vincent.stehle@intel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>