powerpc/boot/install.sh: Fix shellcheck warnings

Fix shellcheck warning such as
"Double quote to prevent globbing and word splitting." and
Use $(...) notation instead of legacy backticks `...`.

Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250518044107.39928-4-maddy@linux.ibm.com