archiso_pxe_http: Fix location of airootfs.md5

Currently, when booting via HTTP, the airootfs.md5 file is downloaded to
/run/archiso/bootmnt/arch/. However, the checksum file is later assumed
to be located at "/run/archiso/bootmnt/arch/${arch}". Fix the _curl_get
invocation and directly place the file in the right directory.

Fixes a regression introduced in commit b3e1d313 ([archiso] Rework
checksum function, 2014-06-28).

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>

archiso/initcpio/hooks/archiso_pxe_http

@@ -37,7 +37,7 @@ archiso_pxe_http_mount_handler () {
     _curl_get "${archiso_http_srv}${archisobasedir}/${arch}/airootfs.sfs" "/${arch}"
 
     if [[ "${checksum}" == "y" ]]; then
-        _curl_get "${archiso_http_srv}${archisobasedir}/${arch}/airootfs.md5" "/"
+        _curl_get "${archiso_http_srv}${archisobasedir}/${arch}/airootfs.md5" "/${arch}"
     fi
 
     mkdir -p "/run/archiso/bootmnt"