Mega-Kang from mkasick
Enable and fix kexec syscall support.

Use mem_text_write_kernel_word when assigning reboot_code_buffer parameters
to avoid protection faults (writes to read-only kernel memory) when
CONFIG_STRICT_MEMORY_RWX is enabled.

Enable caching and buffering for all of physical RAM in the decompressor.

Old method is to enable caching and buffering only for the 256 MB at the
start of the decompressor image.  This makes a hardboot-kexec'd kernel very
slow to decompress since the decompressor is located far above the kernel
destination.  This patch reduces boot time from 35 to 8 seconds on epicmtd.

Support copying of kernel tagged list (atags) in the decompressor.

This is needed to hardboot kexec a kernel with a new tags list (including a
new kernel command line), since the new atags would otherwise be lost with
the limited kernel memory mapping.  Without this patch, a hardboot-kexec'd
kernel uses the atags provided by the bootloader.

Support hard booting to a kexec kernel.

See KEXEC_HARDBOOT config option help for details.

Restrict slave mountspace so Dalvik apps can mount system-wide volumes

Android 4.2 implements multi-user storage using per-process mount
namespaces.  Everything under "/" (the entire filesystem hierarchy) is
marked as a recursive-slave mountspace for all zygote instances.  This is
done so that user-storage sandbox mounts under /storage/emulated are hidden
from other apps and users.

Unfortunately this means that any Dalvik app (actually, any program whose
clone/fork ancestry includes a Dalvik zygote, which is everything except
services spawned directly from init) cannot mount system-wide volumes.
Thus, apps like CifsManager are effectively broken in Android 4.2, since
its cifs mounts are only visible to the CifsManager app itself.  All other
apps see empty mountpoints instead of the mounted volume.  Furthermore,
Linux provides no provision for a process to "escape" a recursive-slave
mountspace in versions prior to Linux 3.8 (setns syscall).

Here, we restrict rootfs-slave calls to /storage (and, due to a possible
kernel bug, /mnt/shell/emulated) so that Dalvik apps can mount system-wide
volumes elsewhere (with appropriate permission, as in earlier versions of
Android), while retaining full multi-user storage compatibility.  It is
made available as a kernel-based workaround for instances where libdvm can
not be modified.

This change requires that a tmpfs volume is mounted as /storage in init.rc.
If this volume is unavailable, then we falls back to the previous behavior
of marking the entire filesystem hierarchy as slave.

*** Do not commit to CyanogenMod Project kernels, as a similar workaround
    for this problem is patched in Zygote/Dalvik/libdvm instead.  ***

Clear download mode flag on kexec hardboot.