Support out-of-source kernel builds

[Werkov]

Disclaimer: the out-of-source builds work just fine for me in the recent version (e0d44a2).

It is only slightly misleading that I set KERNEL_SRC=$BUILD_DIR.
By quickly skimming rapido cuts, KERNEL_SRC is used for build artifacts most of the time. But some cases would need review.

Proposal: introduce a new config variable pointing explicitly to kernel build artifacts.

• In-source builds (nothing changes, user configures a single path)
  • KERNEL_BUILD=KERNEL_SRC
• Out-of-source builds (review if separate path is necessary)
  • KERNEL_BUILD=
  • KERNEL_SRC= -- is it needed?

[ddiss]

I don't normally use out-of-source builds, so can't really judge whether this would be useful to others - my only real concern with adding support would be that rapido.conf configuration becomes even more complicated. We currently have:

# Path to Linux kernel source. Prior to running "rapido cut", the kernel
# should be built, with modules installed (see KERNEL_INSTALL_MOD_PATH below).
# If KERNEL_SRC is not set then rapido will attempt to boot the local kernel
# present under /boot/.
#
# e.g. KERNEL_SRC="/home/me/linux"
#KERNEL_SRC=""

...which IMO is pretty clear that it needs to be built in-place.