Reduce USART1 tx buffer by 8 bytes to fix aligment issue

The recent changes to the timers increased memory by 8 bytes.
   and should have ONLY added 8 bytes
   was  20000dc0	40	20000E00
   is:  20000dc8	40	20000E08
   s/b  20000E08       1f3      next symbol

   But for some unknown reason the linker skipped to the next alignment
   of 256 and wasted 246 bytes.

   20000F00     1f3     next symbol

   Even with .align 8 in the .S file and . = ALIGN(4); in the linker
   script I could not move the allocation back only up to the next
   512 alighment.

   So this is a hack to shift things back 8 bytes.

nuttx-configs/px4io-v2/nsh/defconfig

@@ -767,7 +767,7 @@ CONFIG_USART1_SERIAL_CONSOLE=y
 # USART1 Configuration
 #
 CONFIG_USART1_RXBUFSIZE=64
-CONFIG_USART1_TXBUFSIZE=64
+CONFIG_USART1_TXBUFSIZE=56
 CONFIG_USART1_BAUD=115200
 CONFIG_USART1_BITS=8
 CONFIG_USART1_PARITY=0