esp32/esp32_common.cmake: Clean up RISC-V directives.

This commit cleans up a couple of RISC-V specific directives in the
build script.  Namely, removes the forced inclusion of the "riscv"
component and introduces proper mpy-cross flags.

The "riscv" component is already included by the ESP-IDF build
framework, as certain low-level components would not build otherwise, so
there is no need to add it to the required components list.

The architecture flag for mpy-cross is now set for RISC-V targets, as it
was previously set only for Xtensa targets (and it relied on a string
comparison rather than using the appropriate configuration variable).

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

ports/esp32/esp32_common.cmake

@@ -20,7 +20,6 @@ if(CONFIG_IDF_TARGET_ARCH_RISCV)
         ${MICROPY_DIR}/shared/runtime/gchelper_native.c
         ${MICROPY_DIR}/shared/runtime/gchelper_rv32i.s
     )
-    list(APPEND IDF_COMPONENTS riscv)
 endif()
 
 if(NOT DEFINED MICROPY_PY_TINYUSB)
@@ -237,8 +236,10 @@ idf_component_register(
 set(MICROPY_TARGET ${COMPONENT_TARGET})
 
 # Define mpy-cross flags, for use with frozen code.
-if(CONFIG_IDF_TARGET_ARCH STREQUAL "xtensa")
+if(CONFIG_IDF_TARGET_ARCH_XTENSA)
     set(MICROPY_CROSS_FLAGS -march=xtensawin)
+elseif(CONFIG_IDF_TARGET_ARCH_RISCV)
+    set(MICROPY_CROSS_FLAGS -march=rv32imc)
 endif()
 
 # Set compile options for this port.