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 b3e1d31 ([archiso] Rework
checksum function, 2014-06-28).
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>