readme: initial version

README

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+Prerequisites
+=============
+
+A cross-compiler targetting ARMv6 or ARMv7-M such as the GNU ARM Embedded Toolchain is required to build the source code for the Trusted Boot Module.
+Either install it using your package manager or download the toolchain from https://developer.arm.com/open-source/gnu-toolchain/gnu-rm.
+For Gentoo users, an ebuild is available in the tbm-overlay repository.
+Building your own cross-compiler using a tool like crossdev can be quite tricky, and is therefore discouraged.
+
+To build the source code in this repository, you will also need libopencm3.
+Download the source code for libopencm3 and build it as follows:
+
+	git submodule init
+	git submodule update
+	make -C libopencm3
+
+Supported targets
+=================
+
+The targets that are currently supported are:
+
+ * A simulation compatible with your host device (not up-to-date; requires an older git revision): TARGET=host make
+ * STM32F0 Discovery board: TARGET=stm32f0 make
+ * Trusted Boot Module (rev. 1): TARGET=stm32f1 make
+
+Building
+========
+
+To build the code for the Trusted Boot Module, run:
+
+	TARGET=stm32f1 make
+
+Flashing and Debugging
+======================
+
+In case of the STM32F0 Discovery board, simply connect the device using a USB cable. In case of the TBM, connect the device using SWD.
+Then run the following as root:
+
+	TARGET=stm32f1 make openocd
+
+To flash and run the program, issue the following command:
+
+	TARGET=stm32f1 make run