bootwrapper: Add README

Add a short README file describing what the bootwrapper is
and how to run it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

README.txt

@@ -0,0 +1,74 @@
+boot-wrapper: Start Linux kernels under ARM Fast Models
+
+The boot-wrapper is a fairly simple implementation of a boot loader
+intended to run under an ARM Fast Model and boot Linux.
+
+License
+=======
+
+The boot-wrapper is generally under a 3 clause BSD license
+(see LICENSE.txt for details). Note that some source files
+are under similar but compatible licenses. In particular
+libfdt is dual-license GPL/2-clause-BSD.
+
+Compilation
+===========
+
+The expected method of building is to cross-compile on an
+x86 box. You'll need an ARM cross-compiler. On Ubuntu you
+can get this by installing the packages:
+ gcc-4.6-arm-linux-gnueabi binutils-arm-linux-gnueabi
+ libc6-armel-cross linux-libc-dev-armel-cross gcc-arm-linux-gnueabi
+ libc6-dev-armel-cross cpp-arm-linux-gnueabi
+
+The boot-wrapper can be compiled in two ways:
+ (1) as a small standalone binary which uses the model's semihosting
+ ABI to load a kernel (and optionally initrd and flattened device tree)
+ when you run the model
+ (2) with a specific kernel and initrd compiled into the binary;
+ this is less flexible but may be useful in some situations
+
+For case (1) you can just run:
+ make CROSS_COMPILE=arm-linux-gnueabi- semi
+which will build "linux-system-semi.axf".
+(As with a Linux kernel cross-compile, the CROSS_COMPILE
+variable is set to the prefix of the cross toolchain.
+"arm-linux-gnueabi-" matches the prefix used by the Ubuntu
+cross toolchain.)
+
+For case (2) you'll need a Linux kernel tree to hand; the
+boot-wrapper makefile will automatically look into it to
+extract the kernel. By default this tree is assumed to be in
+"../linux-kvm-arm". Assuming you have that tree set up and
+have built a kernel in it, you can run:
+ make CROSS_COMPILE=arm-linux-gnueabi-
+which will build "linux-system.axf".
+
+You can configure the makefile system by copying config-default.mk
+to config.mk and editing it. This is only likely to be useful for
+case (2); see the comments in config-default.mk for more information.
+
+Running
+=======
+
+To run a model with a linux-system-semi.axf:
+
+RTSM_VE_Cortex-A15x1 linux-system-semi.axf -C cluster.cpu0.semihosting-cmd_line="--kernel /path/to/zImage [--initrd /path/to/initrd] [--dtb /path/to/dtb] [-- kernel command line arguments]"
+
+The paths to the kernel, initrd and device tree blob should all be
+host filesystem paths. The initrd and dtb are both optional. Any text
+following '--' is passed to the kernel as its command line; this is
+also optional.
+
+You may also want to pass other options to the model (for instance
+to enable networking); these are not described here. See the Fast
+Models documentation for more information.
+
+Running a linux-system.axf is the same, except that since all
+the files are built in there's no need to pass a command line:
+
+RTSM_VE_Cortex-A15x1 linux-system.axf
+
+Passing a command line to linux-system.axf is allowed, and any
+kernel/initrd/dtb/commandline specified will override the compiled-in
+version.