50-dracut.install: some tidying

Signed-off-by: Nowa Ammerlaan <nowa@gentoo.org>

Author: Nowa-Ammerlaan

hooks/50-dracut.install

@@ -8,9 +8,10 @@
 
 ver=${1}
 img=${2}
-initrd="${INSTALLKERNEL_STAGING_AREA:-$(dirname "${img}")}/initrd"
-uki="${INSTALLKERNEL_STAGING_AREA:-$(dirname "${img}")}/uki.efi"
-tmp="${INSTALLKERNEL_STAGING_AREA:-$(dirname "${img}")}/dracut-tmp"
+basedir=$(dirname "${img}")
+initrd=${INSTALLKERNEL_STAGING_AREA:-${basedir}}/initrd
+uki=${INSTALLKERNEL_STAGING_AREA:-${basedir}}/uki.efi
+tmp=${INSTALLKERNEL_STAGING_AREA:-${basedir}}/dracut-tmp
 
 # familiar helpers, we intentionally don't use Gentoo functions.sh
 die() {