boot/at91bootstrap: disable PIE and stack-protector build flags

The toolchain wrapper automatically adds Position Independent
Execution and stack protector flags in the build process when selected
in the configuration. at91bootstrap being freestanding code, it
doesn't support these, so we have to disable them.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 1351222486b93b72c00057fb3401d40967025164)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/at91bootstrap/at91bootstrap.mk

@@ -32,8 +32,14 @@ endef
 AT91BOOTSTRAP_POST_PATCH_HOOKS += AT91BOOTSTRAP_APPLY_CUSTOM_PATCHES
 endif
 
+# The at91bootstrap Makefile doesn't support customizing
+# CFLAGS/LDFLAGS, so we cheat and pass our custom flags through CC and
+# LD.
 define AT91BOOTSTRAP_BUILD_CMDS
-	$(MAKE1) CROSS_COMPILE=$(TARGET_CROSS) -C $(@D)/$(AT91BOOTSTRAP_MAKE_SUBDIR)
+	$(MAKE1) CROSS_COMPILE=$(TARGET_CROSS) \
+		CC="$(TARGET_CC) -fno-stack-protector" \
+		LD="$(TARGET_CC) -fno-PIE" \
+		-C $(@D)/$(AT91BOOTSTRAP_MAKE_SUBDIR)
 endef
 
 define AT91BOOTSTRAP_INSTALL_IMAGES_CMDS