Makefile: move SED definition into the main Makefile

Makefile: move SED definition into the main Makefile

Since commit f71a621d91ec27f175fc84012962f88b1107305f, we are using the
SED variable in the main Makefile. However, this variable is only
defined in package/Makefile.in, which gets included only when a
configuration is defined.

This means that, if you do:

 $ make menuconfig savedefconfig

without a configuration defined, it fails with:

/bin/bash: /BR2_DEFCONFIG=/d: No such file or directory
Makefile:898: recipe for target 'savedefconfig' failed
make[1]: *** [savedefconfig] Error 127

This issue affects users of the "buildroot-submodule" project, which
does menuconfig+savedefconfig automatically. They worked around this
issue in commit
https://github.com/Openwide-Ingenierie/buildroot-submodule/commit/d12676b608a58529c6b551aa176464166a200428,
but really "make menuconfig savedefconfig" should work out of the box.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>