Makefile: fix newline escaping for CONFIG_DEFAULT_ENV_FILE

I wanted this to be compatible with mkenvimage, including the ability
to embed newlines in variables by escaping them. But I failed to check
that it works more than once.

Fixes: f3d8f7dd73 (Allow providing default environment from file)
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

Makefile

@@ -1700,7 +1700,7 @@ define filechk_defaultenv.h
 	(grep -v '^#' | \
 	 grep -v '^$$' | \
 	 tr '\n' '\0' | \
-	 sed -e 's/\\\x0/\n/' | \
+	 sed -e 's/\\\x0/\n/g' | \
 	 xxd -i ; echo ", 0x00" ; )
 endef