[timvideos#387] add linux rootfs rebuild instructions/config

software/linux-rootfs/README

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+Steps to rebuild a rootfs image with buildroot:
+
+(1) clone buildroot:
+
+    git clone https://github.com/buildroot/buildroot
+    cd buildroot
+
+(2) configure build
+
+    cp ../litex_vexriscv_defconfig .config
+    make olddefconfig
+    make
+
+(3) the resulting image is in output/images

software/linux-rootfs/litex_vexriscv_defconfig

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+#
+# Target options
+#
+BR2_riscv=y
+BR2_RISCV_32=y
+
+#
+# Instruction Set Extensions
+#
+BR2_riscv_custom=y
+BR2_RISCV_ISA_CUSTOM_RVM=y
+BR2_RISCV_ISA_CUSTOM_RVA=y
+BR2_RISCV_ISA_CUSTOM_RVC=n
+BR2_RISCV_ABI_ILP32=y
+
+# GCC
+BR2_GCC_VERSION_8_X=y
+BR2_GCC_VERSION="8.3.0"
+
+# System
+BR2_TARGET_GENERIC_GETTY=y
+BR2_TARGET_GENERIC_GETTY_PORT="console"
+
+# Filesystem
+BR2_TARGET_ROOTFS_CPIO=y
+
+# Kernel headers
+BR2_KERNEL_HEADERS_VERSION=y
+BR2_DEFAULT_KERNEL_VERSION="5.0.13"
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_0=y
+
+# Rootfs overlay
+BR2_ROOTFS_OVERLAY="../rootfs_overlay"
+
+# Extra packages
+#BR2_PACKAGE_DHRYSTONE=y
+#BR2_PACKAGE_MICROPYTHON=y
+#BR2_PACKAGE_SPIDEV_TEST=y
+#BR2_PACKAGE_MTD=y
+#BR2_PACKAGE_MTD_JFFS_UTILS=y
+

software/linux-rootfs/rootfs_overlay/etc/profile

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+export PATH="/bin:/sbin:/usr/bin:/usr/sbin"
+
+if [ "$PS1" ]; then
+	if [ "`id -u`" -eq 0 ]; then
+		export PS1='\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\# '
+	else
+		export PS1='\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '
+	fi
+fi
+
+export PAGER='/bin/more'
+export EDITOR='/bin/vi'
+
+# Source configuration files from /etc/profile.d
+for i in /etc/profile.d/*.sh ; do
+	if [ -r "$i" ]; then
+		. $i
+	fi
+done
+unset i