EFI: don't call printf after ExitBootServices, since it uses Boot Services

[bcran]

Summary

ExitBootServices terminates all boot services including console access. Attempting
to call printf afterwards can result in a crash, depending on the implementation.

So, we need to move any printf statements to before we call bi_load, and remove any that
depend on calling bi_load first.

Test Plan

Tested on i386 (MinnowBoard Turbot): loader now proceeds to the point where it starts executing the kernel
I'm running a "make tinderbox" to verify that arm still builds.

[tsoome]

stand/efi/loader/arch/arm/exec.c

89

It is not really about allocations - loader does still use the pre-allocated heap, but it is about the need to avoid calling Boot Services (note, Runtime Services are ok). And printf() is directly related to BS because we do use SimpleTextOutputProtocol.

[bcran]

stand/efi/loader/arch/arm/exec.c

89

Ah, of course! Thanks, I'll update the comment.

[bcran]

Update comments.