Mention minimally required bootimage version in error message

Author: phil-opp

Changelog.md

@@ -1,3 +1,5 @@
+- Mention minimal required bootimage version in error message when `KERNEL` environment variable is not set.
+
 # 0.5.2
 
 - Remove redundant import that caused a warning

build.rs

@@ -15,7 +15,9 @@ fn main() {
         Ok(kernel) => kernel,
         Err(_) => {
             eprintln!(
-                "The KERNEL environment variable must be set for building the bootloader."
+                "The KERNEL environment variable must be set for building the bootloader.\n\n\
+                If you use `bootimage` for building you need at least version 0.7.0. You can \
+                update `bootimage` by running `cargo install bootimage --force`."
             );
             process::exit(1);
         }