eficonfig: carve out efi_get_next_variable_name_int calls

To retrieve the EFI variable name by efi_get_next_variable_name_int(),
the sequence of alloc -> efi_get_next_variable_name_int ->
realloc -> efi_get_next_variable_name_int is required.
In current code, this sequence repeatedly appears in
the several functions. It should be curved out a common function.

This commit also fixes the missing free() of var_name16
in eficonfig_delete_invalid_boot_option().

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