xbmk: don't use backticks for command substitution

the newer way handles escaped characters better, and it
can be nested more easily. it's also more readable.

personally, i prefer the old way, because it's more
minimalist, but it occurs to me that a lot of people
nowadays don't know about backticks, but they do know
of the modern way.

to make the code more readable, i have modernised it.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/inject.sh

@@ -12,15 +12,15 @@ cv="CONFIG_GBE_BIN_PATH"
 [ -n "$cvxbmk" ] && cv="$cv $cvxbmk"
 [ -n "$cvchk" ] && cv="$cv $cvchk"
 
-eval "`setvars "" archive boarddir IFD_platform ifdprefix tree new_mac \
-    tmpromdir board xchanged $cv`"
+eval "$(setvars "" archive boarddir IFD_platform ifdprefix tree new_mac \
+    tmpromdir board xchanged $cv)"
 
 inject()
 {
 	remkdir "$tmpromdel"
 
 	[ $# -lt 1 ] && err "No options specified"
-	eval "`setvars "" nuke new_mac xchanged`"
+	eval "$(setvars "" nuke new_mac xchanged)"
 
 	archive="$1";
 	new_mac="xx:xx:xx:xx:xx:xx"
@@ -53,7 +53,8 @@ check_release()
 	[ -L "$archive" ] && err "'$archive' is a symlink"
 	e "$archive" f missing && err "'$archive' missing"
 
-	archivename="`basename "$archive"`" || err "Can't get '$archive' name"
+	archivename="$(basename "$archive" || err "Can't get '$archive' name")" \
+	    || err
 	[ -z "$archivename" ] && err "Can't determine archive name"
 
 	case "$archivename" in
@@ -73,7 +74,7 @@ check_target()
 	[ "$board" = "${board#serprog_}" ] || return 1
 	boarddir="$cbcfgsdir/$board"
 
-	eval "`setcfg "$boarddir/target.cfg"`"
+	eval "$(setcfg "$boarddir/target.cfg")"
 	chkvars tree && x_ ./mk -d coreboot "$tree"
 
 	ifdtool="elf/coreboot/$tree/ifdtool"
@@ -104,7 +105,7 @@ readkconfig()
 {
 	x_ rm -f "$xbtmp/cbcfg"
 	fx_ scankconfig x_ find "$boarddir/config" -type f
-	eval "`setcfg "$xbtmp/cbcfg" 1`"
+	eval "$(setcfg "$xbtmp/cbcfg" 1)"
 	setvfile "$@" || return 1; :
 }