efi_loader: image_loader: replace debug to EFI_PRINT

Just for style consistency, replace all the uses of debug() to
EFI_PRINT() in efi_image_loader.c.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>