uboot-mxs: fix compile problems related to OpenSSL

Use the UBOOT_MAKE_FLAGS defined in include/u-boot.mk and do not
overwrite them to compile the host tools against the shipped LibreSSL.
In addition add a patch to fix a compile problem when compiling the
tools against LibreSSL caused by differences in the API between OpenSSL
1.1 and LibreSSL.

This should fix the compile problems seen in build bot from time to time
by not depending on the host libssl-dev package any more but using the
LibreSSL version from OpenWrt.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/boot/uboot-mxs/Makefile

@@ -35,7 +35,7 @@ UBOOT_TARGETS := \
 	mx23_olinuxino \
 	duckbill
 
-UBOOT_MAKE_FLAGS = $(UBOOT_IMAGE)
+UBOOT_MAKE_FLAGS += $(UBOOT_IMAGE)
 
 define Build/InstallDev
 	$(INSTALL_DIR) $(STAGING_DIR_IMAGE)

package/boot/uboot-mxs/patches/002-tools-mxsimage-Support-building-with-LibreSSL.patch

@@ -0,0 +1,35 @@
+From 74e3ace3495b73f6e592e92eca18175cccdb5a24 Mon Sep 17 00:00:00 2001
+From: Hauke Mehrtens <hauke@hauke-m.de>
+Date: Sat, 17 Feb 2018 15:34:19 +0100
+Subject: [PATCH v2] tools/mxsimage: Support building with LibreSSL
+
+The mxsimage utility fails to compile against LibreSSL because LibreSSL
+says it is OpenSSL 2.0, but it does not support the complete OpenSSL 1.1
+interface.
+
+LibreSSL defines OPENSSL_VERSION_NUMBER with 0x20000000L and therefor
+claims to have an API compatible with OpenSSL 2.0, but it does not
+implement OPENSSL_zalloc() and some other functions in its most recent
+version. OpenSSL implements this function since version 1.1.0.
+
+This commit will activate the compatibility code meant for
+OpenSSL < 1.1.0 also for all versions of LibreSSL, if some version of
+LibreSSL will support these functions in the future the version check
+should be adapted.
+
+Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
+---
+ tools/mxsimage.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+--- a/tools/mxsimage.c
++++ b/tools/mxsimage.c
+@@ -26,7 +26,7 @@
+  * OpenSSL 1.1.0 and newer compatibility functions:
+  * https://wiki.openssl.org/index.php/1.1_API_Changes
+  */
+-#if OPENSSL_VERSION_NUMBER < 0x10100000L
++#if OPENSSL_VERSION_NUMBER < 0x10100000L || defined(LIBRESSL_VERSION_NUMBER)
+ static void *OPENSSL_zalloc(size_t num)
+ {
+ 	void *ret = OPENSSL_malloc(num);