Fix bootloader element on ppc

For some reason [1] introduced -m option without ever checking that the
mapping exists. Because there is no grub-ieee1275 mapping anywhere (not
in base, not in bootloader), pkg-map fails. So stop using the mapping in
package-install of grub-ieee1275 on ppc.

There is another patch that tries to solve the same bug by adding the
mapping [2]. I think it is better to undo the breakage introduced in [1]
first, and then, if various distributions have differing names for the
package, introduce various mappings. My reasoning is that at the moment
this element is broken for all ppc64 distributions. This patch would
fix it for some (namely, Ubuntu). Then we can add mappings as tests
are done for other distributions.

[1] Ibca43173c30c2a74a73a2e2d9dd6d6d832c62694
[2] Id2b0f63a7015f883070fd59b79fd96a1c024858a

Change-Id: I8425876c26e9e416c8ce2f53a4e38d26b4208633
Closes-Bug: #1624021

elements/bootloader/finalise.d/50-bootloader

@@ -59,7 +59,7 @@ function install_grub2 {
     if [ -f "/tmp/grub/install" ] ; then
         source /tmp/grub/install
     elif [[ "$ARCH" =~ "ppc" ]]; then
-        install-packages -m bootloader grub-ieee1275
+        install-packages grub-ieee1275
     else
         install-packages -m bootloader grub-pc
     fi