mini-os: include kernel.h from arm/os.h

This is for consistency with x86/os.h and is needed to compile with
lwIP.
mini-os: initial ARM support

Based on an initial patch by Karim Raslan.

On ARM, Mini-OS will boot and display some output on the console.
Tested with:

make XEN_TARGET_ARCH=arm32 CROSS_COMPILE=arm-linux-gnueabihf- \
	CONFIG_TEST=y CONFIG_START_NETWORK=n CONFIG_BLKFRONT=n \
	CONFIG_NETFRONT=n CONFIG_FBFRONT=n CONFIG_KBDFRONT=n \
	CONFIG_CONSFRONT=n CONFIG_XC=n -j4

Signed-off-by: Karim Allah Ahmed <karim.allah.ahmed@gmail.com>
Signed-off-by: Thomas Leonard <talex5@gmail.com>

---

Changes since v5:

- Moved SVC stack to first 16KB of RAM (provides some protection against
  stack overlow corrupting memory).
- Put translation table in second 16KB (simplifies boot code).
- Define physical_address_offset in C, not asm.
- Moved shared_info_page and irqstack to .bss section (smaller image).

Addressed Julien Grall's comments:
- Added comments explaining the lengths when checking FDT properties.
- Fixed (commented) DEBUG macro.
- Drop check for arm,cortex-a9-gic.
- Get hypercall numbers from xen.h.
- Avoid duplicating the definition of _start's virtual address.
- Added paddr_t type to represent physical addresses.

- Require only minimum property lengths (requested by Ian Campbell).

- Moved __bss_start inside section in linker script.
  The linker adds non-listed sections, such as .got, just before .bss.
  This meant that we accidentally zeroed those out too.
mini-os: arm: show registers, stack and exception vector on fault

Signed-off-by: Thomas Leonard <talex5@gmail.com>

---

Exit with SHUTDOWN_crash on BUG or fault (suggested by Ian Campbell).
mini-os: reorganised arch headers

Before, we had:

- include
- include/arm
- include/x86

Where either arm or x86 was added directly to the search path. So
<x86/arch_limits.h> was the same file as <arch_limits.h> on x86.

We now have:

- include
- arch/arm/include
- arch/x86/include

This means that only the second form is now legal. It also simplifies
installation of the headers, since we can now install everything in the
include directory without getting files for other archs.
mini-os: added make install target

This installs libminios.a, the linker script, the headers and a
pkg-config file to locate them.
mini-os: expose gnttab_table, NR_GRANT_FRAMES, etc

Needed by Mirage.
mini-os: make do_hypervisor_callback weak

This is for Mirage, which handles events synchronously by checking for
pending events before blocking.

Signed-off-by: Thomas Leonard <talex5@gmail.com>
mini-os: added stub for printing floats for sprintf, etc

Override the weak __printf_render_float to implement.
mini-os: make start_kernel weak

Possibly we should just move it to test.c. It should be up to
applications which features they want to initialise.