[riscv] Allow for a non-zero link-time address

Using paging (rather than relocation records) will be easier on 64-bit
RISC-V if we place iPXE within the negative (kernel) virtual address
space.

Allow the link-time address to be non-zero and to vary between 32-bit
and 64-bit builds.  Choose addresses that are expected to be amenable
to the use of paging.

There is no particular need to use a non-zero address in the 32-bit
builds, but doing so allows us to validate that the relocation code is
handling this case correctly.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv64/Makefile.sbi

@@ -1,5 +1,13 @@
 # -*- makefile -*- : Force emacs to use Makefile mode
 
+# Set base virtual address to 0xffffffffeb000000
+#
+# This is aligned to a 2MB boundary and so allows 2MB megapages to be
+# used to map the iPXE binary.  The address pattern is also easily
+# recognisable if leaked to unexpected contexts.
+#
+LDFLAGS		+= --section-start=.prefix=0xffffffffeb000000
+
 # Include generic SBI Makefile
 #
 MAKEDEPS	+= arch/riscv/Makefile.sbi