Set the ARCH, CROSS_COMPILE and PATH variables to support Linux kernel
cross compilation.
ARCH is set according to:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/arch?h=v5.2.2
PATH is set to include the cross toolchain path, which is used by the
kernel Makefile.
CROSS_COMPILE is the CROSS_TRIPLE with a dash appended, as the kernel
Makefile uses it to invoke the toolchain utils.
To build the kernel, depending on the config, libssl-dev and libelf-dev
are required. Those are not dealt in this commit as they are not
correlated to the environment variables.
Validated by build the kernel using default kernel config when possible,
otherwise randomly select a defconfig.
<arch> - <defconfig>
arm64 - defconfig
armv5 and armv5-musl - colibri_pxa270_defconfig
armv6 - bcm2835_defconfig
armv7 and armv7a- tegra_defconfig
*x86 - i386_defconfig
mips and mipsel - ath79_defconfig
ppc64le - wii_defconfig
s390x - defconfig
x64 - x86_64_defconfig
* x86 dockcross toolchain does not provide "ld" and compilation fails.
fix#160
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.
