x86/legacy: disable PAE again
commit 961c0eacea ('x86: fix lifting kernel CPU requirements and always
enable PAE') broke some older geode boards such as Soekris net4826.
Hence disable PAE on x86/legacy again in order to still support those
very old non-PAE capable CPUs.
Fixes FS#773 - PAE broke Soekris net4826
Signed-off-by: Daniel Golle <[email protected]>