bitbake: bitbake-layers: make show-cross-depends avoid long path

bitbake: bitbake-layers: make show-cross-depends avoid long path

The "bitbake-layers show-cross-depends" print the absolute path in the
past, now it will print the relative path (relative to the layer dir) by
default, and add the "-f" option to make it print the absolute path.

For example:

$ bitbake-layers show-cross-depends
[snip]
meta-intel/meta-jasperforest/conf/machine/jasperforest.conf requires meta/conf/machine/include/ia32-base.inc
[snip]

$ bitbake-layers show-cross-depends -f
[snip]
/path/to/poky/meta-intel/meta-jasperforest/conf/machine/jasperforest.conf requires /path/to/poky/meta/conf/machine/include/ia32-base.inc
[snip]

[YOCTO #3824]

(Bitbake rev: df217701318d60559be0b10214b883b8ce4f5d2a)

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