Support invoking qemu in command line and fix a test issue

The script uCore_run can now support invoking qemu in a command prompt
environment. The control hotkeys in qemu can be used to exit emulation
or enter qemu monitor, i.e.

    Ctrl-a x    Exit emulation
    Ctrl-a c    Enter qemu monitor

This patch also fixes the always-fail issue when using qemu 1.4.0. The
problem is that using the option '-gdb' twice no longer work.

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
Merge branch 'chenyh/amd64' into mjj/amd64
Remove legacy code in multi-core scheduling

Signed-off-by: Junjie Mao <eternal.n08@gmail.com>
add multiboot header
Add multiboot and ramdisk

1. add multiboot header
2. add ramdisk support (only available when booted with grub2)

Now the system can be booted from Grub2 via PXE network.
This is tested in Virtualbox. Should be ready for real machine.