lib/oe/package_manager: update default rpm config %_prefer_color

* %_prefer_color is used by rpm to determine which color's ELF file
is preferred to be installed.

Here are file colors:
0 is unknown or other
1 is Elf32
2 is Elf64
4 is MIPS64 n32 (this color is added by oe-core's patch)

if default value set to 7, all colors are preferred color, always
be last-in-wins.

For this scenario, when we have 64bits python3 installed first,
then install 32bits python3 later, 64bits python3 will be overwrited,
and sys.path will point to /usr/lib, not /usr/lib64, this may cause
some python3 modules not work. so fixed by remove setting of default
value 7, and use default value 2 of rpm

* other distro like fedora also use the default %_prefer_color 2

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>