at91bootstrap: Disable parallel build

The at91bootstrap build sometimes fail when doing parallel builds.
Build it without parallel build.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

boot/at91bootstrap/at91bootstrap.mk

@@ -31,7 +31,7 @@ AT91BOOTSTRAP_POST_PATCH_HOOKS += AT91BOOTSTRAP_APPLY_CUSTOM_PATCHES
 endif
 
 define AT91BOOTSTRAP_BUILD_CMDS
-	$(MAKE) CROSS_COMPILE=$(TARGET_CROSS) -C $(@D)/$(AT91BOOTSTRAP_MAKE_SUBDIR)
+	$(MAKE1) CROSS_COMPILE=$(TARGET_CROSS) -C $(@D)/$(AT91BOOTSTRAP_MAKE_SUBDIR)
 endef
 
 define AT91BOOTSTRAP_INSTALL_IMAGES_CMDS