routecache: initialize route cache early

Initialize global variables of route cache
before networking subsystem.

This patch fixes chashes observed on EC2:

page fault outside application, addr: 0x0000000000000000
[registers]
RIP: 0x00000000002577c8 <route_cache::lookup(bsd_sockaddr_in*, unsigned int, rtentry*)+1000>
RFL: 0x0000000000010282  CS:  0x0000000000000008  SS:  0x0000000000000010
RAX: 0x0000000000000000  RBX: 0xffffa00005eb0120  RCX: 0xffff800005eb0210  RDX: 0x00000000000000b7
RSI: 0xffff800005eb0120  RDI: 0x0000000000e290a0  RBP: 0xffff80010258fa70  R8:  0x0000000000e290a0
R9:  0x0000000000e290a0  R10: 0x00171fdfe79c2d73  R11: 0x0000000000000000  R12: 0xffff800005eb0210
R13: 0x0000000000e290a0  R14: 0x00171fdfe79c2d73  R15: 0x0000000000000000  RSP: 0xffff80010258f9e0
Aborted

[backtrace]
0x000000000032851f <???+3310879>
0x0000000000329613 <mmu::vm_fault(unsigned long, exception_frame*)+147>
0x00000000003899f9 <page_fault+105>
0x00000000003888e6 <???+3705062>
0x00000000002649a2 <ip_output(mbuf*, mbuf*, route*, int, ip_moptions*, inpcb*)+2754>
0x000000000027761d <tcp_respond+413>
0x000000000026d2a1 <???+2544289>
0x0000000000271908 <tcp_input+2712>
0x0000000000263769 <ip_input+57>
0x0000000000250871 <netisr_dispatch_src+417>
0x000000000024884e <???+2394190>
0x0000000000250871 <netisr_dispatch_src+417>
0x0000000000288030 <???+2654256>
0x000000000039bd68 <xen::xen_irq::do_irq()+376>
0x000000000039befe <???+3784446>
0x00000000003e394a <thread_main_c+26>
0x0000000000389865 <???+3709029>

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
build.mk: replace kernel base include file

with a definition in build.mk, which allows to set different
base addresses for different architectures, avoids the call
to the preprocessor, and makes it visible in the build.mk,
which makes it more accessible for architectures where the
base address might need to be adjusted.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>