Check for ISA extension in startup

- Check for the 'X' extension in misa at startup. If not present, skip
  initialization for the vector core.

Change-Id: I03a1f14498fbb74e08a6d99eac6e596b01c94c25

crt/kelvin_start.S

diff --git a/crt/kelvin_start.S b/crt/kelvin_start.S
index 9ed259a..07fb0ae 100644
--- a/crt/kelvin_start.S
+++ b/crt/kelvin_start.S
@@ -57,8 +57,13 @@
         mv   t6, zero
 
         ###############################################
-        # Zero out all kelvin registers               #
+        # Check for the X extension, and if present,  #
+        # zero out all kelvin registers               #
         ###############################################
+        csrr a0, misa
+        srli a0, a0, 23
+        andi a0, a0, 1
+        beqz a0, vector_zero_end
         vdup.w.x.m v0, zero
         vdup.w.x.m v4, zero
         vdup.w.x.m v8, zero
@@ -77,6 +82,7 @@
         vdup.w.x.m v60, zero
         acset.v v48, v0
         adwinit.v v48, v0
+vector_zero_end:
 
         # Zero out the bss section
         la   a0, __bss_start__