configs:nitrogen{6sx, 6x, 7, 8m}: fix typo in kernel headers version

A typo has been introduced during the last version bump [1].

[1] 00252b101a86ef136fc4afc045ba16324cbccb3b

Fixes:
[nitrogen6sx]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255632
[nitrogen6x]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255635
[nitrogen7]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255638
[nitrogen6m8]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255640

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Gary Bisson <bisson.gary@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

configs/nitrogen6sx_defconfig

@@ -18,7 +18,7 @@ BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y
 
 # Linux headers same as kernel, a 4.14 series
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4.14=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_14=y
 
 # bootloader
 BR2_TARGET_UBOOT=y

configs/nitrogen6x_defconfig

@@ -18,7 +18,7 @@ BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y
 
 # Linux headers same as kernel, a 4.14 series
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4.14=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_14=y
 
 # bootloader
 BR2_TARGET_UBOOT=y

configs/nitrogen7_defconfig

@@ -17,7 +17,7 @@ BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y
 
 # Linux headers same as kernel, a 4.14 series
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4.14=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_14=y
 
 # bootloader
 BR2_TARGET_UBOOT=y

configs/nitrogen8m_defconfig

@@ -18,7 +18,7 @@ BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y
 
 # Linux headers same as kernel, a 4.14 series
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4.14=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_14=y
 
 # DDR training binaries
 BR2_PACKAGE_FREESCALE_IMX=y