libfdt: Fix signedness comparison warnings

This is a combination of upstream libfdt commits to fix warnings about
comparing signed and unsigned integers:
==========
scripts/dtc/libfdt/fdt.c: In function ‘fdt_offset_ptr’:
scripts/dtc/libfdt/fdt.c:137:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if ((absoffset < offset)
...
==========

For a detailed description of the fixes, see the dtc repo:
https://git.kernel.org/pub/scm/utils/dtc/dtc.git/log/?id=73e0f143b73d808

For this patch the commits between 73e0f143b73d8088 and ca19c3db2bf62000
have been combined and adjusted for the slight differences in U-Boot's
libfdt code base.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/scripts/dtc/libfdt/fdt.c b/scripts/dtc/libfdt/fdt.c
index 8e4cce3..28f4e1a 100644
--- a/scripts/dtc/libfdt/fdt.c
+++ b/scripts/dtc/libfdt/fdt.c
@@ -131,16 +131,20 @@
 
 const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int len)
 {
-	unsigned absoffset = offset + fdt_off_dt_struct(fdt);
+	unsigned int uoffset = offset;
+	unsigned int absoffset = offset + fdt_off_dt_struct(fdt);
+
+	if (offset < 0)
+		return NULL;
 
 	if (fdt_chk_basic())
-		if ((absoffset < offset)
+		if ((absoffset < uoffset)
 		    || ((absoffset + len) < absoffset)
 		    || (absoffset + len) > fdt_totalsize(fdt))
 			return NULL;
 
 	if (!fdt_chk_version() || fdt_version(fdt) >= 0x11)
-		if (((offset + len) < offset)
+		if (((uoffset + len) < uoffset)
 		    || ((offset + len) > fdt_size_dt_struct(fdt)))
 			return NULL;
 
@@ -302,9 +306,12 @@
 
 int fdt_move(const void *fdt, void *buf, int bufsize)
 {
+	if (fdt_chk_basic() && bufsize < 0)
+		return -FDT_ERR_NOSPACE;
+
 	FDT_RO_PROBE(fdt);
 
-	if (fdt_totalsize(fdt) > bufsize)
+	if (fdt_totalsize(fdt) > (unsigned int)bufsize)
 		return -FDT_ERR_NOSPACE;
 
 	memmove(buf, fdt, fdt_totalsize(fdt));