env: Rename getenv_hex(), getenv_yesno(), getenv_ulong()

We are now using an env_ prefix for environment functions. Rename these
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/nvedit.c b/cmd/nvedit.c
index 0468a4b..4033d90 100644
--- a/cmd/nvedit.c
+++ b/cmd/nvedit.c
@@ -327,7 +327,7 @@
 	return env_set(varname, str);
 }
 
-ulong getenv_hex(const char *varname, ulong default_val)
+ulong env_get_hex(const char *varname, ulong default_val)
 {
 	const char *s;
 	ulong value;
@@ -690,7 +690,7 @@
  *			found
  * @return the decoded value, or default_val if not found
  */
-ulong getenv_ulong(const char *name, int base, ulong default_val)
+ulong env_get_ulong(const char *name, int base, ulong default_val)
 {
 	/*
 	 * We can use env_get() here, even before relocation, since the