Add info how to fix failed chroot

Docs/CrossBuild.md

@@ -104,6 +104,33 @@ Another way is to create rootfs with all required libs from scratch: https://wik
 
 # Troubleshooting
 
+## `chroot failed: no such file or directory`
+
+Debootstrap might fail with smth like `chroot failed: no such file or directory` during rootfs generation when it needs to continue setup in rootfs. 
+Or same can happen during manual chroot `sudo chroot ./rootfs/<arch> /bin/bash`, which also fails with same error as debootstrap even though `/bin/bash` is present in rootfs.
+
+To fix this first check that `qemu-<arch>-static` is copied from `/usr/bin/` on host to `/usr/bin` in rootfs. If it's not copied, do that manually. 
+
+If this doesn't solve the problem, this means that binfmt was misconfigured for some reason. To verify this try `sudo chroot ./rootfs/<arch> /usr/bin/qemu-<arch>-static /bin/bash`, and if it works, the problem is indeed with binfmt.
+
+1. First check that binfmt is enabled with `cat /proc/sys/fs/binfmt_misc/status`. 
+2. Then check that config for required arch is present with `cat /proc/sys/fs/binfmt_misc/qemu-<arch>` or `update-binfmts --display`, and that this arch is enabled.
+3. Even if both are present and seem correct, there migt be misconfiguration (i.e. binfmt arch config is outdated). To update config follow next steps (example for RISC-V from https://wiki.debian.org/RISC-V#Manual_qemu-user_installation):
+
+```sh
+$ cat >/tmp/qemu-riscv64 <<EOF
+package qemu-user-static 
+type magic
+offset 0
+magic \x7f\x45\x4c\x46\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\xf3\x00
+mask \xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff
+interpreter /usr/bin/qemu-riscv64-static
+EOF
+$ sudo update-binfmts --import /tmp/qemu-riscv64
+```
+
+Now chroot should work.
+
 ## Non-standard system paths
 
 For systems with non-standard paths (like `/lib64/`) `-L<path>` option might not be enough to find libararies, since `-L` specifies search dirs only for explicitly specified libs (i.e. through `-l<libname>` in command line).