external-toolchain: adjust check for uClibc static toolchains

The current check for uClibc toolchain was verifying that a
ld-uClibc.so dynamic loader was present. However, with static-only
uClibc toolchains, this does not work. Instead, we check for an
uClibc-specific header file in the sysroot.

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

toolchain/helpers.mk

@@ -196,7 +196,7 @@ check_uclibc_feature = \
 #
 check_uclibc = \
 	SYSROOT_DIR="$(strip $1)"; \
-	if ! test -f $${SYSROOT_DIR}/lib/ld*-uClibc.so.* ; then \
+	if ! test -f $${SYSROOT_DIR}/usr/include/bits/uClibc_config.h ; then \
 		echo "Incorrect selection of the C library"; \
 		exit -1; \
 	fi; \