classes/base: don't print header if BUILDCFG_HEADER not set

classes/base: don't print header if BUILDCFG_HEADER not set

If we don't want a header printed at the start of task execution (which
we'd prefer not to within the extensible SDK)  we can accomplish that by
clearing BUILDCFG_VARS and BUILDCFG_HEADER, but that was still printing
a load of blank lines. To keep things simple, check if BUILDCFG_HEADER
is set to something before printing a header.

(From OE-Core rev: d896d77c1d0fc56ff7019bfee3cf06fbc3ede8f3)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>