dtc: Address an assortment of portability problems

I've recently worked with a FreeBSD developer, getting dtc and libfdt
working on FreeBSD.  This showed up a number of portability problems
in the dtc package which this patch addresses.  Changes are as
follows:

	- the parent_offset and supernode_atdepth_offset testcases
used the glibc extension functions strchrnul() and strndupa().  Those
are removed, using slightly longer coding with standard C functions
instead.

	- some other testcases had a #define _GNU_SOURCE for no
particular reason.  This is removed.

	- run_tests.sh has bash specific constructs removed, and the
interpreter changed to /bin/sh.  This apparently now runs fine on
FreeBSD's /bin/sh, and I've also tested it with both ash and dash.

	- convert-dtsv0-lexer.l has some extra #includes added.  These
must have been included indirectly with Linux and glibc, but aren't on
FreeBSD.

	- the endian handling functions in libfdt_env.h, based on
endian.h and byteswap.h are replaced with some portable open-coded
versions.  Unfortunately, these result in fairly crappy code when
compiled, but as far as I can determine there doesn't seem to be any
POSIX, SUS or de facto standard way of determining endianness at
compile time, nor standard names for byteswapping functions.

	- some more endian handling, from testdata.h using the
problematic endian.h is simply removed, since it wasn't actually being
used anyway.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/run_tests.sh

@@ -1,6 +1,6 @@
-#! /bin/bash
+#! /bin/sh
 
-. tests.sh
+. ./tests.sh
 
 export QUIET_TEST=1
 
@@ -15,19 +15,19 @@ tot_vg=0
 tot_strange=0
 
 base_run_test() {
-    tot_tests=$[tot_tests + 1]
+    tot_tests=$((tot_tests + 1))
     if VALGRIND="$VALGRIND" "$@"; then
-	tot_pass=$[tot_pass + 1]
+	tot_pass=$((tot_pass + 1))
     else
 	ret="$?"
 	if [ "$ret" == "1" ]; then
-	    tot_config=$[tot_config + 1]
+	    tot_config=$((tot_config + 1))
 	elif [ "$ret" == "2" ]; then
-	    tot_fail=$[tot_fail + 1]
+	    tot_fail=$((tot_fail + 1))
 	elif [ "$ret" == "$VGCODE" ]; then
-	    tot_vg=$[tot_vg + 1]
+	    tot_vg=$((tot_vg + 1))
 	else
-	    tot_strange=$[tot_strange + 1]
+	    tot_strange=$((tot_strange + 1))
 	fi
     fi
 }
@@ -52,7 +52,7 @@ wrap_test () {
 	else
 	    ret="$?"
 	    if [ "$ret" -gt 127 ]; then
-		signame=$(kill -l $[ret - 128])
+		signame=$(kill -l $((ret - 128)))
 		FAIL "Killed by SIG$signame"
 	    else
 		FAIL "Returned error code $ret"