Set the LMA of the riscv kernel to the OpenSBI jump target by default

This allows us to boot FreeBSD RISCV on QEMU using the -kernel command line
options. When using that option, QEMU maps the kernel ELF file to the
addresses specified in the LMAs in the program headers.

Since version 4.2 QEMU ships with OpenSBI fw_jump by default so this allows
booting FreeBSD using the following command line:
qemu-system-riscv64 -bios default -kernel /.../boot/kernel/kernel -nographic -M virt

Without this change the -kernel option cannot be used since the LMAs start
at address zero and QEMU already maps a ROM to these low physical addresses.

For targets that require a different kernel LMA the make variable
KERNEL_LMA can be overwritten in the config file. For example, adding
makeoptions KERNEL_LMA=0xc0200000 will create an ELF file that will be
loaded at 0xc0200000.

Before:
There are 4 program headers, starting at offset 64

Program Headers:

Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
LOAD           0x001000 0xffffffc000000000 0x0000000000000000 0x75e598 0x8be318 RWE 0x1000
DYNAMIC        0x71fb20 0xffffffc00071eb20 0x000000000071eb20 0x000100 0x000100 RW  0x8
GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x0
NOTE           0x693400 0xffffffc000692400 0x0000000000692400 0x000024 0x000024 R   0x4

After:

There are 4 program headers, starting at offset 64

Program Headers:

Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
LOAD           0x001000 0xffffffc000000000 0x0000000080200000 0x734198 0x893e18 RWE 0x1000
DYNAMIC        0x6f7810 0xffffffc0006f6810 0x00000000808f6810 0x000100 0x000100 RW  0x8
GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x0
NOTE           0x66ca70 0xffffffc00066ba70 0x000000008086ba70 0x000024 0x000024 R   0x4

Reviewed By: br, mhorne (earlier version)
Differential Revision: https://reviews.freebsd.org/D23436