Another external toolchain fix

Change the definition of TARGET_LDFLAGS to use --sysroot
$(STAGING_DIR) instead of -L$(STAGING_DIR)/lib
-L$(STAGING_DIR)/usr/lib. It fixes the following failure while trying
to build mtd-utils :

/usr/local/xtools/arm-unknown-linux-uclibcgnueabi/bin/arm-unknown-linux-uclibcgnueabi-gcc -L/home/thomas/local/buildroot-output/build_arm/staging_dir/lib -L/home/thomas/local/buildroot-output/build_arm/staging_dir/usr/lib -o /home/thomas/local/buildroot-output/build_arm/mtd_orig/flash_eraseall /home/thomas/local/buildroot-output/build_arm/mtd_orig/crc32.o /home/thomas/local/buildroot-output/build_arm/mtd_orig/flash_eraseall.o
/usr/local/xtools/arm-unknown-linux-uclibcgnueabi/lib/gcc/arm-unknown-linux-uclibcgnueabi/4.3.2/../../../../arm-unknown-linux-uclibcgnueabi/bin/ld: cannot find /lib/libc.so.0
collect2: ld returned 1 exit status
make[1]: *** [/home/thomas/local/buildroot-output/build_arm/mtd_orig/flash_eraseall] Error 1

At the same time, simplify the definition of TARGET_CFLAGS, because
the -I$(STAGING_DIR)/include -I$(STAGING_DIR)/usr/include
-I$(TOOLCHAIN_EXTERNAL_PATH)/$(TOOLCHAIN_EXTERNAL_PREFIX)/include are
no longer necessary since we sysroot the toolchain in $(SYSROOT_DIR).

This patch has no effect on non-external toolchain builds.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>