scripts: gki: Disable DEBUG_FS for user variant builds

DEBUG_FS is enabled in {target}-gki_defconfig for supporting
internal test teams requirements. But android requirement is
that, DEBUG_FS should be disabled for user builds. So add
support to merge debugfs.config fragment for user builds.

Change-Id: I584db74375ce968fd2eacbe03548f07f73416152
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>