doc: README.mxs: Add instruction to install 'libssl-dev'

Since commit bce883707 (ARM: mxs: tools: Add mkimage support for MXS bootstream)
the following build error is seen when doing a MAKEALL build:

$ ./MAKEALL mx28evk
Configuring for mx28evk - Board: mx28evk, Options: ENV_IS_IN_MMC
mxsimage.c:18:25: fatal error: openssl/evp.h: No such file or directory

Add an entry about the need of installing the 'libssl-dev' package.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>

doc/README.mxs

@@ -63,6 +63,11 @@ copy the binary by hand:
 Make sure the "elftosb" binary can be found in your $PATH, in this case this
 means "/usr/local/bin/" has to be in your $PATH.
 
+Install the 'libssl-dev' package as well. On a Debian-based distribution, this
+package can be installed as follows:
+
+	$ sudo apt-get install libssl-dev
+
 2) Compiling U-Boot for a MXS based board
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