config: unify board names to lowercase

Currently board names are used inconsistently across the project.

* Name of defconfigs for various boards use lowercase.
* Directory of config files for various boards use lowercase.
* CONFIG_BOARD uses uppercase.

This confuses the configuration scripts and leads to unintended overwriting of
.config, as well as missing of board-specific headers during compilation because
the include paths are case-sensitive.

This patch converts the default board names to lowercase to resolve such
issues. Users are still free to define their own boards in either uppercase or
lowercase as long as they keep the cases consistent.

Tracked-On: #2794
Signed-off-by: Junjie Mao <junjie.mao@intel.com>

Author: junjiemao1

hypervisor/arch/x86/configs/apl-up2.config

@@ -1,5 +1,5 @@
 # Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)
-CONFIG_BOARD="UP2"
+CONFIG_BOARD="up2"
 CONFIG_SERIAL_PCI=y
 CONFIG_SERIAL_PCI_BDF="0:18.0"
 CONFIG_COM_IRQ=6

hypervisor/arch/x86/configs/nuc6cayh.config

@@ -1,3 +1,3 @@
 # Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)
-CONFIG_BOARD="NUC6CAYH"
+CONFIG_BOARD="nuc6cayh"
 CONFIG_SERIAL_LEGACY=y

hypervisor/arch/x86/configs/nuc7i7bnh.config

@@ -1,3 +1,3 @@
 # Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)
-CONFIG_BOARD="NUC7i7BNH"
+CONFIG_BOARD="nuc7i7bnh"
 CONFIG_SERIAL_LEGACY=y