kernel/linux: allow headers from full custom source tree

Accept a local tarball name as the source of the Linux kernel headers,
rather than forcing the user to use either an upstream tarball, or a
local pre-installed headers tree.

 --- binutils/ld/ldmain.c

 +++ binutils/ld/ldmain.c

 @@ -293,2 +293,3 @@ main (int argc, char **argv)

    link_info.combreloc = TRUE;

 +  link_info.relro = TRUE;

    link_info.strip_discarded = TRUE;