isoimage-isohybrid: use TRANSLATED_TARGET_ARCH instead of MACHINE_ARCH
isoimage-sihybrid plugin uses MACHINE_ARCH to get the name of initrd image.
It doesn't work for all machines, for example for quemux86-64 machine
MACHINE_ARCH is quemux86_64 and initrd name is
core-image-minimal-initramfs-qemux86-64.cpio.gz
Used TRANSLATED_TARGET_ARCH variable to get the initrd image name.
Replaced MACHINE_ARCH->TRANSLATED_TARGET_ARCH in WICVARS variable to
make it available from <image>.env file.
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
