xbmk: don't call mkdir. use xbmkdir (new function)

xbmkdir checks if a directory exists, before running
mkdir, and then still uses -p

i was testing xbmk on arch linux today, and noticed
that it errored out when a directory already exists.

i'm mitigating against buggy or differently behaving
mkdir implementations this way, by wrapping around
it.

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

include/rom.sh

@@ -19,7 +19,7 @@ buildser()
 		x_ make -C "$sersrc" BOARD=$2
 	fi
 
-	x_ mkdir -p "bin/serprog_$1"
+	x_ xbmkdir "bin/serprog_$1"
 	x_ mv "$serx" "bin/serprog_$1/serprog_$2.${serx##*.}"
 }
 
@@ -119,7 +119,7 @@ check_coreboot_util()
 		return 0
 	fi
 
-	x_ mkdir -p "$utilelfdir"
+	x_ xbmkdir "$utilelfdir"
 	x_ cp "$utilsrcdir/$1" "$utilelfdir"
 
 	if [ "$1" = "cbfstool" ]; then
@@ -453,7 +453,7 @@ cprom()
 
 	printf "Creating new %s image: '%s'\n" "$projectname" "$newrom"
 
-	x_ mkdir -p "bin/$target"
+	x_ xbmkdir "bin/$target"
 	x_ $cpcmd "$irom" "$newrom"
 }