[CORE] Don't use vmm_panic() for failures in init_bootcpu()

When we are in init_bootcpu() there is no VCPU running and
the stack is boot-time stack provided by architecture specific
code.

The vmm_panic() tries to do stacktrace when called from anywhere.
This stacktrace fails from init_bootcpu() on arm64 hardware.

This patch replaces use of vmm_panic() in init_bootcpu() with
vmm_printf() followed by vmm_hang().

Signed-off-by: Anup Patel <anup@brainfault.org>