MIMXRT1050_EVt: keep hyperflash_config/image_vector_table symbols in LTO builds

Add a "used" attribute to hyperflash_config/image_vector_table to fix ARMC6 build with
the "-flto" flag.
(Error: L6236E: No section matches selector - no section to be FIRST/LAST. )

This attribute, attached to a function/variable, means that code must be emitted
for the function even if it appears that the function is not referenced.

Author: maciejbocianski

targets/TARGET_NXP/TARGET_MCUXpresso_MCUS/TARGET_MIMXRT1050/TARGET_EVK/xip/evkbimxrt1050_flexspi_nor_config.c

@@ -17,7 +17,7 @@
  ******************************************************************************/
 #if defined(XIP_BOOT_HEADER_ENABLE) && (XIP_BOOT_HEADER_ENABLE == 1)
 #if defined(__CC_ARM) || defined(__ARMCC_VERSION) || defined(__GNUC__)
-__attribute__((section(".boot_hdr.conf")))
+__attribute__((section(".boot_hdr.conf"), used))
 #elif defined(__ICCARM__)
 #pragma location = ".boot_hdr.conf"
 #endif

targets/TARGET_NXP/TARGET_MCUXpresso_MCUS/TARGET_MIMXRT1050/drivers/fsl_flexspi_nor_boot.c

@@ -14,7 +14,7 @@
 
 #if defined(XIP_BOOT_HEADER_ENABLE) && (XIP_BOOT_HEADER_ENABLE == 1)
 #if defined(__CC_ARM) || defined(__ARMCC_VERSION) || defined(__GNUC__)
-    __attribute__((section(".boot_hdr.ivt")))
+    __attribute__((section(".boot_hdr.ivt"), used))
 #elif defined(__ICCARM__)
 #pragma location=".boot_hdr.ivt"
 #endif