Convert CONFIG_SYS_BOOTM_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOTM_LEN

As part of this, rework error handling in boot/bootm.c so that we pass
the buffer size to handle_decomp_error as CONFIG_SYS_BOOTM_LEN will not
be available to host tools but we do know the size that we passed to
malloc().

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/a5y17lte_defconfig b/configs/a5y17lte_defconfig
index 44915ea..ea0773e 100644
--- a/configs/a5y17lte_defconfig
+++ b/configs/a5y17lte_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PBSIZE=1024
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_DM_I2C_GPIO=y