Comment # 1 on bug 968185 from Jiri Srain

There is no interface, which would allow the boot loader to pass the encryption
key to the Linux kernel (without sacrificing the security).

Since in your partitioning, bootloader needs the password in order to load the
kernel, it has to ask in addition to the kernel asking for password.

While I recognize your request as valid, from my POV it is not a bug, but a
desired behavior. Implementation of kernel interface for passing the encryption
password should be rather handled as a feature request.

As a workaround, you can have a separate /boot partition, which will not be
encrypted.