build: fix auto defconfig rule

build: fix auto defconfig rule

We should only run "defconfig" if ".config" is missing. Commit
317c98cb91 have added a dependency on "tools/fixdep", so make would
start runnning "defconfig" also when "tools/fixdep" is newer than
".config" and thus overwrite any changes made by a developer.
Reintroduce intended behavior of the rule to only generate a default
Kconfig when ".config" is missing.

Fixes: 317c98cb91 ("build: hook kconfig into xen build system")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>