env: Rename getenv/_f() to env_get()

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

Quite a few places use getenv() in a condition context, provoking a
warning from checkpatch. These are fixed up in this patch also.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/Barix/ipam390/ipam390.c b/board/Barix/ipam390/ipam390.c
index 3a58402..d203429 100644
--- a/board/Barix/ipam390/ipam390.c
+++ b/board/Barix/ipam390/ipam390.c
@@ -157,7 +157,7 @@
 	u32 maxcpuclk = CONFIG_DA850_EVM_MAX_CPU_CLK;
 	u32 rev = 0;
 
-	s = getenv("maxcpuclk");
+	s = env_get("maxcpuclk");
 	if (s)
 		maxcpuclk = simple_strtoul(s, NULL, 10);