bitbake: bitbake/cooker: Print which pkgs would be built in -g output

bitbake: bitbake/cooker: Print which pkgs would be built in -g output

This is for giving the user a clear list to show which pkg would be
built, we have the "bitbake -g", but it is not easy to read for people,
it is for "dot". Improve the "bitbake -g" to also save a pn-buildlist:

$ bitbake -g core-image-sato
...
NOTE: PN build list saved to 'pn-buildlist'
[snip]

The contents of pn-buildlist:

busybox
shadow-native
pth
sysfsutils
qemu-helper-native
curl-native
ncurses-native
gdbm
xserver-xorg
linux-libc-headers
[snip]

[YOCTO #2404]

(Bitbake rev: 18aff925aece774d0172894e25584353519ca03f)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>