package_rpm.bbclass: fix the arch (replace "-" with "_")

package_rpm.bbclass: fix the arch (replace "-" with "_")

rpm can't use the "-" as the arch, which causes problem, e.g., when
MACHINE = "beagleboard":

* The arch should be armv7a-vfp-neon, but rpm only takes the armv7a,
  this is incorrect since it is mixed with real arch armv7a.

* The nativesdk's arch should be i686-nativesdk (or x86_64-nativesdk),
  but rpm only takes the i686 (or x86_64), this in incorrect since it is
  mixed with the arch i686 (or x86_64).

Replace "-" with "_" when rpm package and the rootfs generation would
fix the problem, I think this is fine since it doesn't change the tune's
arch, the package manager doesn't care about the arch's name, but it
needs a unify arch system to avoid confusing. This is similar to what we
have done on the deb which fixed the arch i486, i586 and so on to i386.

[YOCTO #2328]

(From OE-Core rev: fc985f511da86400e4fa7d17555216c12eb51666)

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