efi/libstub: Avoid physical address 0x0 when doing random allocation

Ben reports spurious EFI zboot failures on a system where physical RAM
starts at 0x0. When doing random memory allocation from the EFI stub on
such a platform, a random seed of 0x0 (which means no entropy source is
available) will result in the allocation to be placed at address 0x0 if
sufficient space is available.

When this allocation is subsequently passed on to the decompression
code, the 0x0 address is mistaken for NULL and the code complains and
gives up.

So avoid address 0x0 when doing random allocation, and set the minimum
address to the minimum alignment.

Cc: <[email protected]>
Reported-by: Ben Schneider <[email protected]>
Tested-by: Ben Schneider <[email protected]>
Reviewed-by: Ilias Apalodimas <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>

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