Implement "ct-ng" building, add "linux-mips".

Add a mechanism to construct a full cross-compiler environment using the
"crosstool-ng" cross-compiler building utility. This is implemented in
the new "common.crosstool" include, and augments the "dockcross/base"
base image.

Update Makefile to consolidate Dockerfile generation, notably the
"sed"-based inclusion directives.

Finally, employ all of this to generate a "linux-mips" 32-bit hard-float
MIPS cross-compiler Dockcross image.

linux-mips/Dockerfile.in

@@ -0,0 +1,42 @@
+FROM dockcross/base:latest
+
+# This is for 32-bit Big-Endian MIPS devices with hard floating point enabled
+
+#include "common.crosstool"
+
+# The cross-compiling emulator
+RUN apt-get update \
+&& apt-get install -y \
+  qemu-user \
+  qemu-user-static \
+&& apt-get clean --yes
+
+# The CROSS_TRIPLE is a configured alias of the "mips-unknown-linux-gnu" target.
+ENV CROSS_TRIPLE mips-unknown-linux-gnu
+
+ENV CROSS_ROOT ${XCC_PREFIX}/${CROSS_TRIPLE}
+ENV AS=${CROSS_ROOT}/bin/${CROSS_TRIPLE}-as \
+    AR=${CROSS_ROOT}/bin/${CROSS_TRIPLE}-ar \
+    CC=${CROSS_ROOT}/bin/${CROSS_TRIPLE}-gcc \
+    CPP=${CROSS_ROOT}/bin/${CROSS_TRIPLE}-cpp \
+    CXX=${CROSS_ROOT}/bin/${CROSS_TRIPLE}-g++ \
+    LD=${CROSS_ROOT}/bin/${CROSS_TRIPLE}-ld
+
+ENV QEMU_LD_PREFIX "${CROSS_ROOT}/${CROSS_TRIPLE}/sysroot"
+ENV QEMU_SET_ENV "LD_LIBRARY_PATH=${CROSS_ROOT}/lib:${QEMU_LD_PREFIX}"
+
+ENV DEFAULT_DOCKCROSS_IMAGE dockcross/linux-mips
+
+COPY Toolchain.cmake ${CROSS_ROOT}/
+ENV CMAKE_TOOLCHAIN_FILE ${CROSS_ROOT}/Toolchain.cmake
+
+# Build-time metadata as defined at http://label-schema.org
+ARG BUILD_DATE
+ARG IMAGE
+ARG VCS_REF
+ARG VCS_URL
+LABEL org.label-schema.build-date=$BUILD_DATE \
+      org.label-schema.name=$IMAGE \
+      org.label-schema.vcs-ref=$VCS_REF \
+      org.label-schema.vcs-url=$VCS_URL \
+      org.label-schema.schema-version="1.0"