Makefile: Fix support for external platforms

Makefile: Fix support for external platforms

Set `KCONFIG_EPLAT_DIRS` to the list of external platforms `$(P)` instead
of `$(E)`, which is not defined in this context.

With the change, the `config-submenu.sh` script will pickup the `Config.uk`
of an external platform as expected.

Signed-off-by: Robert Kuban <robert.kuban@opensynergy.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #1082