tools/Makefile: get openssl CFLAGS from pkg-config

Fixes building mkimage on systems where OpenSSL header files do not
live in the standard include path.

Signed-off-by: Pierre Bourdon <delroth@gmail.com>
diff --git a/tools/Makefile b/tools/Makefile
index d377d85..12a3027 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -150,6 +150,8 @@
 
 # MXSImage needs LibSSL
 ifneq ($(CONFIG_MX23)$(CONFIG_MX28)$(CONFIG_ARMADA_38X)$(CONFIG_ARMADA_39X)$(CONFIG_FIT_SIGNATURE),)
+HOSTCFLAGS_kwbimage.o += \
+	$(shell pkg-config --cflags libssl libcrypto 2> /dev/null || echo "")
 HOSTLOADLIBES_mkimage += \
 	$(shell pkg-config --libs libssl libcrypto 2> /dev/null || echo "-lssl -lcrypto")