eficonfig: refactor file selection handling

eficonfig_select_file_handler() is commonly used to select the
file. eficonfig_display_select_file_option() adds an additional
menu to clear the selected file.
eficonfig_display_select_file_option() is not always necessary
for the file selection process, so it must be outside of
eficonfig_select_file_handler().

This commit also renames the following functions to avoid confusion.
 eficonfig_select_file_handler() -> eficonfig_process_select_file()
 eficonfig_select_file() -> eficonfig_show_file_selection()
 eficonfig_display_select_file_option() -> eficonfig_process_show_file_option()

Finally, test_eficonfig.py need to be updated to get aligned with
the above modification.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>