Make the libcrypt test somewhat cross-compilation compliant

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

Index: b/src/Makefile
===================================================================
--- a/src/Makefile
+++ b/src/Makefile
@@ -78,7 +78,7 @@
 endif
 
 # Additional libs for GNU libc.
-ifneq ($(wildcard /usr/lib*/libcrypt.a),)
+ifneq ($(wildcard $(SYSROOT)/usr/lib*/libcrypt.a),)
   SULOGINLIBS	+= -lcrypt
 endif