mini-os: Fixed shutdown thread

Before, it read "" and started a shutdown immediately. Now, it waits for
a non-empty value and then actually shuts down.

Signed-off-by: Thomas Leonard <talex5@gmail.com>
mini-os: Tidied up code

Signed-off-by: Karim Allah Ahmed <karim.allah.ahmed@gmail.com>
[talex5@gmail.com: separated from big ARM commit]
Signed-off-by: Thomas Leonard <talex5@gmail.com>
mini-os: Initial commit to port minios to ARM

On ARM, Mini-OS will boot and display some output on the Xen emergency
console (only visible if Xen is compiled in debug mode). Tested with:

make XEN_TARGET_ARCH=arm32 CROSS_COMPILE=arm-linux-gnueabihf- \
  CONFIG_TEST=n CONFIG_START_NETWORK=n CONFIG_BLKFRONT=n \
  CONFIG_NETFRONT=n CONFIG_FBFRONT=n CONFIG_KBDFRONT=n \
  CONFIG_CONSFRONT=n CONFIG_XC=n

Signed-off-by: Karim Allah Ahmed <karim.allah.ahmed@gmail.com>
[talex5@gmail.com: made x86_64 support work again]
[talex5@gmail.com: split into multiple patches]
Signed-off-by: Thomas Leonard <talex5@gmail.com>
mini-os: Added FDT support on ARM

Signed-off-by: Karim Allah Ahmed <karim.allah.ahmed@gmail.com>
[talex5@gmail.com: split out FDT support into a separate patch]
[talex5@gmail.com: fixed "make clean" for FDT]
Signed-off-by: Thomas Leonard <talex5@gmail.com>