Cosmetic: Fix a number of typos, no functional changes.

Fix various misspellings of things like "environment", "kernel",
"default" and "volatile", and throw in a couple grammar fixes.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
diff --git a/include/common.h b/include/common.h
index 8addf43..6aa2398 100644
--- a/include/common.h
+++ b/include/common.h
@@ -383,7 +383,7 @@
 /**
  * setenv_addr - Set an environment variable to an address in hex
  *
- * @varname:	Environmet variable to set
+ * @varname:	Environment variable to set
  * @addr:	Value to set it to
  * @return 0 if ok, 1 on error
  */