efi_loader: Moved the generated ESL file to objtree

Tom reports that generating the ESL file we need for authenticated
capsule updates fails to work on azure which expects a RO git tree.

Move it to $(objtree)

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
3 files changed