[linux] Allow a sysroot to be specified via SYSROOT=...

[linux] Allow a sysroot to be specified via SYSROOT=...

The cross-compiler will typically use the appropriate sysroot
directory automatically.  This may not work for toolchains where a
single cross-compiler is used to produce output for multiple CPU
variants (e.g. 32-bit and 64-bit RISC-V).

Add a SYSROOT=... parameter that may be used to specify the relevant
sysroot directory, e.g.

 make CROSS=riscv64-linux-gnu- SYSROOT=/usr/riscv32-linux-gnu/sys-root \
      bin-riscv32-linux/tests.linux

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