Reintroduce the possibility to force an architecture

Among other things, 36459f3 ([archiso] Drop aitab support, 2014-06-28)
removed the possibility to manually set a specific architecture by using
kernel parameters. This, however, is useful, e.g. when installing Arch
Linux on a device that reports itself as i586 but works fine with the
i686 flavor.

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

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
[archiso] Update README

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@gmail.com>
[archiso] Disable ext4 resize_inode on airootfs.

Remove unused feature, just to save space
sizeof ext4 image (empty): 4.3M vs 403K (du airootfs.img)
sizeof used space (empty):  48M vs  20K (df)

Thanks Christian Hesse for initial tip.

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@gmail.com>