Fix 64-bit booke kernel builds after the ldscript changes

Commits r326203 and r326978 broke 64-bit booke kernels by introducing a 1MB
zero-pad between the ELF header and the start of the kernel. This didn't
cause a build failure, but caused kernels to need to be loaded into memory
1MB lower, which could easily break scripts expecting previous behavior.
This change matches the similar change made to AIM in r327358.