Properly disable devtools in AOT mode

[sdeleuze]

As pointed out by @jamesward here, a build with the AOT plugin is broken when spring-boot-devtools is added.

The spirit of this commit was to say we don't enable AOT mode by default so you can use devtools and IDE integration by default.

As discussed with @bclozel we should probably:

• Ignore the devtools dependency during the AOT generation
• Remove the check that fails the build when devtools is detected
• Document that devtools won't be active in AOT mode as it is not when the app is ran as a JAR
• Maybe add a runtime warning log when devtools is detected when AOT mode is enabled

[jamesward]

I ran into this again when running: ./gradlew bootBuildImage

Execution failed for task ':generateAot'.
> Could not generate spring.factories source code

Removing developmentOnly("org.springframework.boot:spring-boot-devtools") gets it going again.

[sdeleuze]

@jamesward I created #1579 related issue.