If the Kconfig option is not set, there's no effect on code size.
If set, .bss increases by 256 bytes (the buffer) and .text by
another 200 bytes.

This adds timestamping support, so we know the duration of the
interrupt (because we found it's an important thing when problems
occur).
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>