mkimage: Build signing only if board has CONFIG_FIT_SIGNATURE

At present mkimage is set up to always build with image signing support.
This means that the SSL libraries (e.g. libssl-dev) are always required.

Adjust things so that mkimage can be built with and without image signing,
controlled by the presence of CONFIG_FIT_SIGNATURE in the board config file.

If CONFIG_FIT_SIGNATURE is not enabled, then mkimage will report a warning
that signing is not supported. If the option is enabled, but libraries are
not available, then a build error similar to this will be shown:

lib/rsa/rsa-sign.c:26:25: fatal error: openssl/rsa.h: No such file or directory

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/config.mk b/config.mk
index 5a91203..81c2584 100644
--- a/config.mk
+++ b/config.mk
@@ -96,7 +96,6 @@
 HOSTLDFLAGS	+= $(call os_x_before, 10, 5, "-multiply_defined suppress")
 else
 HOSTCC		= gcc
-HOSTLIBS	+= -lssl -lcrypto
 endif
 
 ifeq ($(HOSTOS),cygwin)
@@ -211,6 +210,11 @@
 LDFLAGS_FINAL += --gc-sections
 endif
 
+# TODO(sjg@chromium.org): Is this correct on Mac OS?
+ifdef CONFIG_FIT_SIGNATURE
+HOSTLIBS	+= -lssl -lcrypto
+endif
+
 ifneq ($(CONFIG_SYS_TEXT_BASE),)
 CPPFLAGS += -DCONFIG_SYS_TEXT_BASE=$(CONFIG_SYS_TEXT_BASE)
 endif