scripts/Makefile.headersinst: install headers from scratch file

If headers_install is executed from a deep/long directory structure, the
shell's maximum argument length can be execeeded, which breaks the operation
with:

| make[2]: execvp: /bin/sh: Argument list too long
| make[2]: ***

By dumping the input files to a scratch file and using xargs to read the
input list from the scratch file, we can avoid blowing out the maximum
argument size and install headers in a long path name environment.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>