ArmPlatformPkg/Bds: stop inputting more characters when string is full

ArmPlatformPkg/Bds: stop inputting more characters when string is full

If EditHIInputStr() is called, say with a MaxCmdLine of 2, the user is
currently allowed to enter 2 characters.

If the second character is a carriage return/line feed, this is
substituted with a NULL and the function returns.

If the second character is a regular character, the loop terminated and
the function returns.  However, the buffer has not been NULL terminated.

This patch prevents the user from entering a regular character as the
final character and ensures that the only way out of the input is by
pressing ESC or ENTER (or equivalent).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15325 6f19259b-4bc3-4df7-8a09-765794883524