efi_loader: export efi_get_configuration_table

In multiple places we need a function to find an EFI configuration table.
Rename get_config_table() to efi_get_configuration_table() and export it.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>