Android kernel startup entry point

source code path:
/kernel/arch/arm/kernel/head.S

1. Just called from the decompressor code.
2. Precondition:

• MMU = off
• D-cache = off
• I-cache = don't care.
• r0 = 0
• r1 = machine nr, 
• r2 = atags or dtb pointer

3. Position independent.
    If we link kernel @ 0xC0008000, we call this at __pa(0xc008000)