package/mtd: fix build with libexecinfo

Fix the following build failure raised on uclibc and musl since the
addition of libexecinfo package in commit
eea8ba446c10701a273432552108d80fb2224ef4:

/home/buildroot/autobuild/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-musleabihf/10.3.0/../../../../arm-buildroot-linux-musleabihf/bin/ld: tests/fs-tests/integrity/integck-integck.o: in function `check_failed.constprop.0':
integck.c:(.text+0x96c): undefined reference to `backtrace'

Fixes:
 - http://autobuild.buildroot.org/results/9409ebf6517db87672f3f4df5909a8ac4abfe8af

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/mtd/mtd.mk

@@ -13,6 +13,15 @@ MTD_CPE_ID_VENDOR = mtd-utils_project
 MTD_CPE_ID_PRODUCT = mtd-utils
 MTD_INSTALL_STAGING = YES
 
+MTD_LDFLAGS = $(TARGET_LDFLAGS)
+
+ifeq ($(BR2_PACKAGE_LIBEXECINFO),y)
+MTD_DEPENDENCIES += libexecinfo
+MTD_LDFLAGS += -lexecinfo
+endif
+
+MTD_CONF_ENV += LDFLAGS="$(MTD_LDFLAGS)"
+
 ifeq ($(BR2_PACKAGE_MTD_JFFS_UTILS),y)
 MTD_DEPENDENCIES += zlib lzo host-pkgconf
 MTD_CONF_OPTS += --with-jffs