Rename getenv_r() into getenv_f()

While running from flash, i. e. before relocation, we have only a
limited C runtime environment without writable data segment. In this
phase, some configurations (for example with environment in EEPROM)
must not use the normal getenv(), but a special function.  This
function had been called getenv_r(), with the idea that the "_r"
suffix would mean the same as in the _r_eentrant versions of some of
the C library functions (for example getdate vs. getdate_r, getgrent
vs. getgrent_r, etc.).

Unfortunately this was a misleading name, as in U-Boot the "_r"
generally means "running from RAM", i. e. _after_ relocation.

To avoid confusion, rename into getenv_f() [as "running from flash"]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>

README

@@ -2572,7 +2572,7 @@ to save the current settings.
 
 Please note that the environment is read-only until the monitor
 has been relocated to RAM and a RAM copy of the environment has been
-created; also, when using EEPROM you will have to use getenv_r()
+created; also, when using EEPROM you will have to use getenv_f()
 until then to read environment variables.
 
 The environment is protected by a CRC32 checksum. Before the monitor