golang: add option to skip building bootstrap

golang: add option to skip building bootstrap

Introduce a new configuration option - GOLANG_BUILD_BOOTSTRAP - that
defaults to enabled:

- When enabled, build the local bootstrap instead of using an external
  one for host Go. If GOLANG_EXTERNAL_BOOTSTRAP_ROOT is set, skip the
  initial 1.4 step.

- When disabled, let Go auto-detect the toolchain or use the toolchain
  specified in GOLANG_EXTERNAL_BOOTSTRAP_ROOT to skip building the
  bootstrap and build host Go using that toolchain directly. Configured
  toolchain must be compatible with the current version of Go.

HOST_BUILD_DEPENDS cannot be conditional, so some Makefile juggling is
needed.

Put menu config under bootstrap only.

Remove unnecessary package-related flags from host-only bootstrap.

Signed-off-by: George Sapkin <redacted>