installer: Increase size of mount for iso due to size increase

installer: Increase size of mount for iso due to size increase

- download.sh defines a mount of 512M for downloading the iso for use with PXE.
   Unfortunately since CU192 the iso sizes have been greater than 512M
- This was identified by a user on the forum as their PXE install failed to work.
- They had suggested increasing it to around 800M. As CU200 is already at 658M I
   thought it made more sense to give a bit more room and so specified 1024M. If this
   is considered too excessive then it can always be modified.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>