kbuild: refactor headers_* targets in Makefile

o Use lower case for local variables
o Add a helper target for common targets
o Use $(hdr-inst)= ... to make Make invocations simpler
o Add -rR to make invocations

In total this adds more lines than it removes but the
benefit is better readability

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: David Woodhouse <dwmw2@infradead.org>