Currently, callers of fdt_next_tag() must usually follow the call with
some sort of call to fdt_offset_ptr() to verify that the blob isn't
truncated in the middle of the tag data they're going to process.
This is a bit silly, since fdt_next_tag() generally has to call
fdt_offset_ptr() on at least some of the data following the tag for
its own operation.

This patch alters fdt_next_tag() to always use fdt_offset_ptr() to
verify the data between its starting offset and the offset it returns
in nextoffset.  This simplifies fdt_get_property() which no longer has
to verify itself that the property data is all present.

At the same time, I neaten and clarify the error handling for
fdt_next_tag().  Previously, fdt_next_tag() could return -1 instead of
a tag value in some circumstances - which almost none of the callers
checked for.  Also, fdt_next_tag() could return FDT_END either because
it encountered an FDT_END tag, or because it reached the end of the
structure block - no way was provided to tell between these cases.

With this patch, fdt_next_tag() always returns FDT_END with a negative
value in nextoffset for an error.  This means the several places which
loop looking for FDT_END will still work correctly - they only need to
check for errors at the end.  The errors which fdt_next_tag() can
report are:
	- -FDT_ERR_TRUNCATED if it reached the end of the structure
	   block instead of finding a tag.

	- -FDT_BADSTRUCTURE if a bad tag was encountered, or if the
           tag data couldn't be verified with fdt_offset_ptr().

This patch also updates the callers of fdt_next_tag(), where
appropriate, to make use of the new error reporting.

Finally, the prototype for the long gone _fdt_next_tag() is removed
from libfdt_internal.h.

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