cmake: emu: Don't require ARMFVP_BIN_PATH environment variable

Enhance the armfvp emulator cmake handling to allow additional options
for setting ARMFVP_BIN_PATH, relaxing the previous requirement to set in
an environment variable. This makes it possible to run twister with
multiple armfvp platforms located in different paths.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>