x86: trampoline cleanup

x86: trampoline cleanup

To make future changes less error prone, and to slightly simplify a
possible future conversion to a relocatable trampoline even for the
multiboot path (pretty desirable given that we had to change the
trampoline base a number of times to escape collisions with firmware
placed data),
- remove final uses of bootsym_phys() from trampoline.S, allowing the
  symbol to be undefined before including this file (to make sure no
  new references get added)
- replace two easy to deal with uses of bootsym_phys() in head.S
- remove an easy to replace reference to BOOT_TRAMPOLINE

Signed-off-by: Jan Beulich <jbeulich@novell.com>