[arch][riscv32] rename the qemu target sifive-e

The initial port is really a Sifive E platform. Call it what it is and
make space for bringing up the Sifive U and virt qemu target.

target/sifive-e/rules.mk

@@ -0,0 +1,11 @@
+LOCAL_DIR := $(GET_LOCAL_DIR)
+
+GLOBAL_INCLUDES += $(LOCAL_DIR)/include
+
+PLATFORM := sifive
+VARIANT := sifive_e
+
+MEMSIZE ?= 0x4000     # 16KB
+
+#include make/module.mk
+