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/uniphier_v8_defconfig b/configs/uniphier_v8_defconfig
index db986cf..eccfb0d 100644
--- a/configs/uniphier_v8_defconfig
+++ b/configs/uniphier_v8_defconfig
@@ -16,6 +16,7 @@
 CONFIG_PREBOOT="env exist ${bootdev}preboot && run ${bootdev}preboot"
 CONFIG_LOGLEVEL=6
 CONFIG_CMD_CONFIG=y
+CONFIG_SYS_BOOTM_LEN=0x2000000
 # CONFIG_CMD_XIMG is not set
 CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS=10
 CONFIG_CMD_GPIO=y